It really isn't hard to ride your bike 50, 60 or 70 miles every day, it just takes a bit of preparation to condition yourself. Ride your bike at every opportunity and do increasingly long weekend rides. Start with 20 or 30miles and increase to 50 or 60 miles. Push your self and try to keep your speed up. On tour there's usually no hurry so when you back off to an easy pace 50 miles will seem like a gentle spin of the legs. When I tour I don't push myself and just keep the speedo at around 12mph and I find that I can do 50 miles before lunch and arrive at my destination early enough to take things easy in the afternoon.