Sonder Bikes
A guide to choosing the best rucksack for your next trail run
When you're covering serious distance on foot (whether that's a multi-hour trail run, a mountain marathon, or a fastpacking adventure across wild terrain), what you carry on your back matters intensely. Every gram, every strap placement, every pocket position affects how freely you can move and how efficiently you can run. Get the pack wrong, and it becomes a constant distraction. Get it right, and it disappears, leaving you focused entirely on the trail ahead.
This guide covers everything you need to know about choosing and fitting a running pack for long-distance efforts, from understanding what makes running packs different to selecting the right capacity for your adventures.
Why running packs are different from hiking packs
A running pack isn't simply a lighter version of a hiking pack: it's an entirely different category of kit, engineered around fundamentally different movement patterns and priorities.
Body-hugging fit. While hiking packs create structure and airflow through rigid back panels, running packs prioritise close body contact. The pack needs to move with you, not independently of you. This means softer materials, more flex, and designs that conform to your torso rather than sitting away from it.
Bounce elimination. The repetitive impact of running amplifies any instability in your pack. What feels like minor movement while walking becomes maddening bouncing at pace. Running packs use wraparound designs, multiple adjustment points, and close-fitting harnesses to lock the load against your body and eliminate vertical movement.
Easy-access storage. Stopping to retrieve items from your pack breaks rhythm and costs time. Running packs position key storage (nutrition, water, phone) on shoulder straps and hip belts where you can reach them without slowing down. The best designs let you eat, drink, and check your route without breaking stride.
Weight distribution for running. Running posture differs from hiking posture. Your centre of gravity shifts forward, your arms pump, your torso rotates. Running packs sit higher and closer than hiking packs, keeping weight centred where it won't disrupt your natural running mechanics.
Choosing the right capacity
The right pack size depends on what you're carrying, how long you'll be out, and what the event or route demands.
5-10L for trail runs
For trail runs up to a few hours, particularly in good conditions, a small vest-style pack or minimalist running pack provides enough capacity for water, nutrition, a lightweight layer, and essentials. This category suits runners who want to move fast with minimal load: the racing approach.
15-20L for long trail days
Longer days (think 6+ hours) require more capacity for additional food, layers for changing weather, mandatory safety kit, and potentially a headtorch if there's any chance of finishing in the dark. Packs in this range offer the sweet spot of capacity without unnecessary bulk.
20-30L for fastpacking and mountain marathons
When your running extends into multi-day territory (fastpacking trips, stage races, or events like the Marathon des Sables), you need serious capacity without sacrificing the running-specific fit. This is where packs like the Idris 25L come into their own: enough volume for overnight kit, spare clothing, substantial food supplies, and mandatory equipment, while still wearing like a race vest rather than a hiking pack.
For detailed guidance on matching pack size to activity, see: What Size Backpack Do I Need?
Essential features for running packs
Look for these design elements that distinguish running packs from hiking packs.
Flexible back systems
Traditional hiking packs use structured foam panels and frames to create load-bearing capability and ventilation. Running packs take a different approach, prioritising flexibility over structure, body conformity over airflow.
The most effective running pack designs use soft, continuous foam that wraps around your back and hips as a single unit. This creates a body-hugging fit that moves with your running motion rather than fighting against it. Less rigid support, more freedom of movement: a deliberate trade-off that makes sense when you're running rather than walking.
Shoulder strap pockets
On-the-go accessibility defines the running pack experience. Look for multiple pockets on the shoulder straps themselves, sized for soft flasks, gels, your phone, and small essentials. The best designs let you refuel without stopping, keeping everything you need within arm's reach throughout your run.
Hydration strategy
Most running packs offer multiple hydration options: a sleeve for a traditional bladder, side pockets sized for water bottles, and soft flask pockets on the shoulder straps. Consider your preference: some runners like the hands-free convenience of a bladder and hose, others prefer the tactile feedback of knowing exactly how much water remains in a bottle.
For very long efforts, having capacity for both gives you flexibility to carry more fluid when water sources are scarce.
Roll-top vs zip access
Roll-top closures offer significant advantages for running: they let the pack compress as you eat through food and shed layers, maintaining a snug fit rather than leaving empty space that can shift around. A half-empty pack with a fixed zip closure tends to bounce more than a compressed roll-top that shrinks with its contents.
Roll-tops also provide an extra layer of weather protection for the main compartment, useful when you're caught out in mountain weather.
The Idris 25L: purpose-built for ultra running
The Idris 25L was developed with input from ultra runners tackling the world's toughest mountain marathons. At 590g with 25 litres of capacity, it hits the sweet spot for multi-day events and fastpacking adventures.
Why it's different from hiking packs
Where most packs in the range use structured foam panels with ventilation channels, the Idris takes a deliberately different approach. A single piece of soft, continuous foam wraps from back to hips, enclosed in reverse mesh that flexes with your running motion. This sacrifices the load-bearing rigidity of a hiking pack for the body-conforming flexibility that ultra running demands.
As we explained in our article on The Evolution of Pack Fit: "The Idris breaks from the moulded panel approach entirely—less support, more freedom. Exactly what ultra runners need."
Race-vest fit in a pack
Updated shoulder strap construction improves fit around the chest and where the straps meet the pack body. The result feels more like a race vest than a traditional backpack: close, stable, and designed specifically for the running motion. Dual chest straps and adjustable hip belt let you dial in the fit precisely.
Capacity for the long haul
The 25-litre capacity handles everything you need for multi-day mountain marathons: mandatory kit, spare clothing, substantial food supplies, hydration, and emergency equipment. The roll-top closure expands when you're loaded up at the start and compresses as you work through supplies, keeping the fit optimised throughout. Oliver confirms: "Brilliant Backpack. Comfy when running ~50km in the hills. Carries everything you need for two days really well." Roland adds practical experience: "I've had the rucksack for 3 weeks and have been on multiple 6, 9 and 18km trail runs with it loaded up to 5.4kg total pack weight. Does what you need." Tom summarises: "Perfect for Fastpacking/Mountain Marathons. A well thought out design with plenty of features, at an excellent price."
Organisation for on-the-move access
The Idris puts storage where you need it:
- Large stretch mesh front pocket for quick-stash layers
- Dual side pockets for water bottles
- Internal hydration sleeve for a bladder
- Shoulder strap pockets for nutrition and essentials
- Hip belt pockets for on-the-go snacks
- Trekking pole attachment loops
- Rear light attachment point for low-visibility conditions
What to pack for ultra running
Packing for ultra running balances mandatory requirements with weight consciousness.
Mandatory kit for mountain marathons
Most mountain marathons and ultra events require you to carry specific mandatory equipment. Check your race requirements carefully—common items include:
- Waterproof jacket with taped seams
- Emergency foil blanket or bivvy
- Whistle (note: the Idris sternum strap includes an integrated whistle)
- Map of the course
- Mobile phone
- Headtorch with spare batteries
- First aid kit
- Minimum food and water quantities
Nutrition and hydration
Fuelling strategy varies by runner, but as a baseline: aim for 200-300 calories per hour for sustained efforts, and at least 500ml of fluid per hour (more in hot conditions). Pack more than you think you'll need: bonking 30km from the finish is not where you want to discover you under-catered.
Emergency essentials
Beyond mandatory kit, consider:
- Blister kit and tape
- Electrolyte tablets
- Cash or card
- Personal medication
- Emergency contact details
Clothing layers
Mountain weather changes fast. Even in summer, carrying a lightweight insulation layer and waterproof gives you options when conditions turn. Arm warmers and a buff provide warmth-to-weight efficiency without bulk.
Fitting your running pack properly
A running pack should feel like part of your body: secure, stable, and moving with you rather than independently.
Eliminating bounce
Start by loading the pack with your actual kit (fit varies significantly between an empty pack and a loaded one). Put the pack on and tighten the hip belt first, positioning it around your natural waist. Then adjust the shoulder straps until the pack sits close to your back without gaps.
Set both chest straps (if your pack has two) to different heights and tighten until secure but not restrictive. You should be able to breathe deeply without constraint.
Adjusting on the move
Go for a test run before committing to your setup. Any bounce or shift becomes obvious within the first few minutes of running. Make micro-adjustments to chest strap tightness and shoulder strap tension until the pack feels locked in place.
Avoiding chafing
Chafing emerges over distance, often in unexpected places. Pay attention to where straps cross seams in your clothing, where buckles might rub, and where sweat accumulates. Mesh-backed foam, like the Idris uses, helps reduce friction against skin, but lubricant on high-friction areas remains good insurance for very long efforts.
Running pack vs running vest: which do you need?
Running vests and running packs serve different purposes, though the lines between them have blurred.
Running vests (typically 5-12L) prioritise minimal weight and maximum accessibility. They work brilliantly for races with aid stations, shorter trail runs, and fast-moving days where you don't need to carry much.
Running packs (typically 15-30L) provide the capacity for self-sufficiency, carrying your own food, water, and equipment for extended periods without resupply. They suit stage races, fastpacking, mountain marathons, and any situation where you need to be fully self-contained.
The Idris 25L bridges this gap: pack capacity with vest-like fit. For runners whose adventures regularly exceed what a vest can handle, it eliminates the compromise between carrying enough and carrying comfortably.
Making your choice
The right running pack fits so well you forget it's there, holds everything you need without excess bulk, and keeps key items accessible while you're moving. For shorter efforts with aid station support, a smaller vest may suffice. For genuine self-sufficiency over long distances (fastpacking trips, mountain marathons, multi-day stage races), capacity matters.
The Idris 25L was built specifically for this longer, more demanding end of running. Race-vest fit in a 25-litre pack, developed with ultra runners and tested on the world's toughest courses. At 590g, it adds minimal weight while providing genuine multi-day capability.
Whatever distance you're tackling, fit the pack properly, test it before race day, and trust your preparation. The trail awaits.
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