when i was 18 (1973), i did an unsupported tour from rhode island to near mexico city and then back as far as north carolina, from mid august to mid october. i remember getting to the border in laredo, texas and, with my rudimentary math, figured i had done about 2100 miles in 21 days. no cycling computer, just estimating my milage from maps, so just how accurate that figure was is anyone's guess. what i averaged after that point is also unknown.

i also did 212 miles the first day, which probably contributed to a swollen ankle i had by the time i reached relatives in alexandria, virginia, where i spent 3 days i think. i also spent a couple of days visiting relatives in north carolina. other than that, i basically rode from morning to night and didn't think 120 days were particulary difficult. on the other hand, i was riding alone, not particularly into sightseeing, and am the type that gets antsy with nothing to do. summer days are long. on the other -otherhand, i wasn't in any hurry either. i did take a day off when a hurricane came through in louisiana, and i spent 3 or 4 days in monterey, mexico. on regular rainy days i probably did 70 - 80 miles, spending a lot of time reading & drying out in restaurants over coffee. i recall another deluge kind of day in mexico somewhere, walking my bike through ankle-deep-in-water long sections of road and getting practially nowhere before i eventually gave up and retired to a cantina for the rest of the day.

i think that if you are careful about not over-doing it the first week or so, you shouldn't have any real problems averaging 100 miles per day and still have several hours of "relax" time everyday.

best of luck on your big adventure