I have to politely disagree with staehpj1. I notice that on his TransAm he averaged just shy of 60 miles per day - - 73 days, 4244 miles. Similar for his other trips.
Yes I think my TA was actually about 58 miles per day. Not sure how similar my other trips were though. They were actually kind of all over the board at 38, 38, 53, 73, and 74 mpd if you count all days (one of the 38's would be a 43 if I didn't count a 5 day stay in Yosemite). I don't think anything about that says much about how feasible an 80 mile per day TA would be for the OP especially since he is 24 years younger and a triathlete who has done an ironman (actually the OP only need to average 75 mpd to finish in his allotted time). Bear in mind that the TA was my first trip and I was over packed, that I am not a very good athlete, that I turn 64 this year, and that I typically ride very little when not on a tour.
I probably do more hiking and exploring than staehpj1 does while on tour - backcountry up to the Pacific Crest in Yosemite, crossing the Grand Canyon and picking up my bike on the other side, hiking up to the ancient bristlecone forest in Great Basin.
Yep, I am sure that you do and that is great. The thing is that I don't think that riders who are focused on riding something like the TA are typically doing very many long hikes.
Then there are the big breakdowns which I hope you never have - but if you tour long enough you probably will. I can think of three offhand - when you are in Fumbuck, Arkansas miles from nowhere. I had a derailleur snap. I had a seat post snap. (Which makes for some rather uncomfortably cycling) And I had a wheel rim split. If you care close enough to a big town with a bike shop, you can limp along. But if you are 150 miles away from anything, you may just need to call Performance, FedEx it, and wait.
Maybe, but then again if "150 miles away from anything" with a rim split, it might be just as likely that the rider would hitchhike the 150 miles, particularly if they have a deadline.