You could visit your local Holland&Barrett food shop and see which types of meal-replacement and food bars they sell (eg Powerbars). Then buy one of everything and eat your way through them until you identify the ones you like (or failing that, until you find one that you could tolerate for 48 hours!)
Go back and buy a dozen of the one(s) you like most.
Do the same with an isotonic drink-powder and buy some that are already packed in individual sachets.
Seal 48-hours worth of bars, powder and water purifying tablets in zip-lock bags.
Job done, you'll probably find the pack has a shelf-life of at least one-two years and can be rotated with your usual outdoor stuff easily for a low-cost low-tech option.
If you're anything like me, you'll also need something caffeine-based. I find I can get by on a few sachets of the instant coffee drinks (instant coffee, milk and sugar already mixed, available from all the usual supermarkets) if its just for a limited period (and assuming you can heat water)
I recently completed a 60 mile route over a 24-hr period, so I can vouch for this feeding method!