Before my first tour two years ago I just tried to ride my bike as much as possible, which isn't too much with a job and all. I put the panniers on and filled them with cloths and a 2 leter water bottle and just rode. During the week I did a 10 mile loop in the hills above my house. On weekends I would try to do at least 20 miles a day and the most I did was 34. On days I had no time to ride I'd try to get on the stationary bike for 30 minutes.
So the tour comes and the first day is 64 miles. The second day is 84 miles and on and on. The toatal milage for any given day was always way over my longest ride of 34 miles that I did at home. Once you are on your tour, that is your job, get from point A to point B, once you do that you're finished for the day. I found it completely different than my daily rides.
I've done three tour since the first and the training seems to be the same 10 miles after work and 20 on weekends, yet the tours are all 60+ days for 10 days or so. The hardest was a trip in February as it gets dark early so I can't ride here during the week so the first few days were really hard, but by day seven I managed to do 108 miles.