If you can ride 70 miles for 3 consecutive days with only moderate aches and pains, you're good to go. Make sure any saddle issues are settled.

Normally, I'd say just be in reasonably good cycling shape, and train on up as you tour. But, with a group, that's really not feasible.

As for the bike, Enthusiast is spot on. Alternatively, for a dedicated tourer in that price range, look at the Novara Safari or the Windsor Tourist.