It’s a question often asked… But it’s the wrong start point!
Start with a few questions to assess your needs before beginning to look at specific packs:
What will you have to carry (be honest – there’s nothing worse than not being able to get everything in!)?
What is on the minimum kit list?
What will you need to reach easily, and what can be stored for emergencies only?
A good way to get an idea of volume is to pack everything into a known volume drybag. That way you know how much space you need. But always add a little extra…we did one event that started at -1 degC and finished at +30; there was a lot of extra clothing stashed by the end of that one!
Do you want bottles, soft flasks, bladder or combination of?
Straws or not? Hose routing over or under?
Bottles high, low or a choice?
Will you be topping up in aid stations? Do you need your bladder to be easily accessible (without taking everything out of the pack!)
Are you using/carrying poles?
If so how do you prefer to stow them?
Horizontal, Vertical, Front, Back, All of the above!?
How much food are you carrying?
What type (gels, solids etc)
How accessible do you want it?
Are you stocking up during the event?
Is it route marked or do you need easy access to map and compass?
Are you carrying a GPS?
Do you need to carry an emergency beacon?
Measure your chest size with minimal (summer) kit on, and worst case kit.
Make sure you buy a pack which covers your size ‘range’.
Armed with your list of features…
…research what packs have the features you need…be prepared to become obsessed…and be prepared to try more than one! This can become an expensive hobby so if you are starting out we’d recommend looking at second hand or some of the cheaper (but still great quality brands) like Decathlon or Harrier. Start this process early in your training and experiment and finesse during your long runs, making sure you can access hydration, food and the kit you need quickly and easily and without irritation – you should learn if you don’t get along with bladder hoses on a long run instead of on the big day…
As soon as you get your pack, before removing the labels(!), make sure it fits. Load it with everything you will be carrying (inc liquid) and whilst you are wearing your warm layers. Also make sure it still fits when the bladder/bottles are empty and you are in just a T shirt.
One final thought, if this is your first event…do you want/need a pack that can be used for your next, slightly bigger adventure? Because it will happen!!