Sounds like you are giving it serious thought. Having read a few comments I would add the following,
1 Bear in mind technical problems. You only have to have one split tyre whihc may take a few hours of finding a bike shop and you will have lost half a day.
2. Two tents will be logistically a lot easier. Also if one or two of your group drops out you can continue with one smaller tent
3. Do not underestimate time spent setting up tent and packing up. It takes ages! Also plan ahead for food if doing yr own cooking. Think about clothes washing. Not washing anything either means very smelly clothes (and much worse, infected shorts which can be nasty) or taking tons of stuff. Laundrettes once in while can be good. perhaps stop in a town and do a wash at lunchtime whilst eating? Stuff spun in machine but not tuble dried will dry on strapped on the panniers
4 If you can stealth camp (hard in a big group) you can do longer days as you will have to continue close to sunset to avoid detection, and in the morning you have not go the time wasting attractions of proper toilet and washing facilities.
5 For training, no substitute for tons of riding. I would try to do 3 x 110 laden miles on the trot in advance. Even if you do it from home, carry all your kit, set up tent each night in the garden, cook food you bought on route and wash your clothes in a bucket with cold water. See how you cope. Also do as many short rides as you can to get your body used to the saddle and iron out niggles.
6 set off with the bike in perfect condition
Best of luck