I'm fairly active, although I'm going to curtail my workouts a bit. I imagine heavy labour or adventuring on an empty stomach would be a bit of a challenge, but we could assume that the average fantasy hero would be a bit younger and tougher than me. Of course you would want an army behind you, but most quests assume that is not possible or desirable. Anyway - a week. You gorge yourself, carry some biltong to chew on, catch the odd opportunistic meal. No big deal.