I stay fairly fit all the time (able to ride 30 miles without much trouble or discomfort afterwards) but have trouble getting in a ride even once a week during school year (I'm a teacher.) Prior to a tour I try and ride as much as possible, but it doesn't always happen. Some years I start my tour after having ridden quite a bit (i. e. 2-4 times a week over 3 or 4 weeks) and some years I start after not having been on the bike for a month! (The end of my school year is often quite busy.)

No matter how in-shape I am, it always takes a few days to get acclimated to the demands of being on tour. The first day I typically start wishing I'd get to the campground after about 20 miles!

Knowing this, I plan my itinerary with short days at first: 20-35 miles is plenty. I'm one who tends to mosy anyway, rather than being determined to grind out a high mile count every day. Sometimes I've even taken a rest day on day 3 or 4 to recharge, relax, and enjoy after the "shock" of the first couple of days. After those first few days I usually fall into a routine of averaging about 50 miles a day, with 60-70 mile days being common.

I've read lots of accounts of people riding themselves into shape on tour. My advice would be to allow for this by having modest goals for the first few days, even a rest day if you need it. Furthermore, I suggest your goal should always be to enjoy the experience, rather than to achieve a certain average miles-per-day. Of course, if getting a certain number of miles in every day is what you enjoy, go for it.