As far as your mileage goes, it is fairly high. I have crossed the country 5 times plus tons of other tours. For me personally, I have found anything over 65 miles per day (overall average, not riding average) makes for "all riding, no play" and drops the enjoyment factor noticeably. If you have not done tours with that high of mileage, the extra 10-15 miles per day (AVERAGE) really add to the riding days mileage.
You also mention you wanted to be done in 80 days @ 60mpd, or 4,800 miles. Isn't your route closer to 5,500 miles or 69mpd?
Overall, start as early in the spring as you can tolerate and enjoy a pace so if you want to stop and/or not do high mileage you can. After all, you are retired so enjoy the time.