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. And when you ride up to the Yukon and Alaska, there are just some days where it is best not to set out at all - esp. if you have the advantage of some kind of shelter. So maybe, I use more off days - but I think you have to plan for some.
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.
Also, there are unexpected glitches that can happen at the beginning or end of your trip. The worst thing if you fly or Amtrak it is for your bike not to show up on the baggage carousel. That has happened to me once - when it went to the wrong city. Took a day and a half to get it. (Read the small print in your baggage "contract". Nice airlines may offer to cover some of your costs, but they don't have to.)
I stick by my math - 400 miles per week (66 per day + a day off) if you are a moderate cyclists. 450 miles per week (75 per day + a day off) if you are moving at a good clip. Check out the journals at CrazyGuy and see how many people did the TransAm + Western Express (3762 miles) in less than 8 weeks.
My first cross-country trip was from Astoria, OR to the Outer Banks of NC - used a more direct route than the TransAm, so let's say about 3600 miles in 10 weeks. I was 31 at the time, commuted regularly 25 miles round-trip and had taken a few weekend rides prior to that. I stopped to smell the flowers at various national parks and other places, but 8 weeks would have been pushing it.