My approach is to simply not plan to ride further than I can comfortably ride on the tour. If 60 miles wipes you out, then maybe that's your limit for now. As the tour progresses, it'll get easier.

It's all part of my aversion to exercise for the sake of exercise. I don't see any reason to train for things, when I can just stay flexible and only ride as far as I feel like going on any given day.

Also, it's not a race, pace yourself. 10mph = 16km/h, which isn't far off my normal biking pace anyway - when not tired, I'll probably run around 20km/h on the flats. Touring is a slow-and-steady endeavor. Take your time, smell the roses.