A friend of mine did the Pacific Crest Trail (2650 miles) two years ago at age 54. It took him 98 days and about 30 min. He went ultralight (of course), resupplied about every 5 days. Did 27 miles a day, every day. Started at 170 lbs. Finished at 140 lbs. The thing about doing it that applies here is timing. On the PCT, you need to deal with blistering heat in the lower portion and heavy snowfall in the mountains, which gives you a specific window of opportunity. Going Mexico to Canada (S to N) creates a larger window. Anyway, timing is something to definitely look at when hiking through northern latitudes.
This same guy just did a bicycle ride, Canada to Mexico (56 yrs old). Upon retiring from his job in 3 1/2 years, he is going to sail his boat (which he built himself and lives on in the Bahamas) around the world. He's not much of a "couch potato". In fact, I don't think he has a couch.
Pete