I would also add Xan, that its not just about fitness. You will be travelling in a (relatively) remote area, on some big hills, where you need to be self sufficient, and have good riding skills, some good skills and good judgement and some downright common sense. You are not lecking about in the woods, with help a quick mobile call away!
You need to be able to:
Judge your riding abilities, to avoid falls (especially at excessive speed).
Fix anything(ish) that goes wrong with your bike – you need knowledge, tools and spares.
Fix you or a mate if you hurt yourselves – some basic fist aid knowledge and kit.
Navigate on hills / paths with few or no markers, and know where you are at all times.
Understand basics of mountain weather, and when to go and when to stay in the cafe.
Have enough kit/water/food to stay comfy and safe in the first place, and to sit it out/bale out if it goes belly up.
Understand that in the hills, being miserable and p*ss*d off, is all part of the adventure 🙂
Its a great adventure – go for it and enjoy! (and post the pics up here!)