Then I started my first tour. I fought it for a couple weeks, then relaxed as I averaged 10-11 mph, and rode 30-50 miles a day (in the Appalachians). After a while I made it up to 60-70 miles a day. My body could tolerate that kind of miles if it wsn't too hilly. My overall average was a bit over 50 miles a day. In subsequent tours, I'm comfortable with that kind of mileage.
You'll get guesses all over the place, from 30-40 to 80-100 miles a day. YMMV. If you're young, or if you're going to ride very light, you may be able to hit (or exceed!) my pre-tour goals. It might be a good idea to plan a few weekends, or even a week-long tour, to see how you fare.