You don't need to " train" specifically to do this. The "ride and recover" advice is good, but applies mainly to those who are incurring high levels of training stress in order to get faster. If you are prepared to take your time, successive days of 70 miles are not a problem. I've often toured at an average of c. 60 miles per day for weeks at a time, and because I've just been bimbling along, I have found it very comfortable.