I think it's pretty different for everyone. Do you have kids? I expect having to juggle a couple or more would be tougher.
My DS was 6 and then 7 for my two trips. DW and I are not the most fit people, but we do the parks like crazy. Mind you, I expect I put the most mileage on my feet since I'm the one always going for stuff (fast passes, trips to the bathroom, drinks).
Anyways, we've gone twice - 14 nights and 15 nights. Every day we're at a park for rope drop (or EMH) and sometimes stay until close. We've never scheduled a day for downtime and we never left the park early.
I think the secret for us was a nice sit down meal break twice per day to keep us going. But we were certainly tired and sore at the end of the trip.
Everyone I talk to thinks we're crazy, but the ones that have gone tell us about all the time they spent in "downtime" and how they had really wished they'd gotten everything they wanted to do. Us, we get all Disney parks (including water parks) plus Universal's two and Seaworld and go back for seconds.
We're going back in the fall for our third trip. We're squeezing in Busch Gardens this time in 10 days because we're doing a cruise at the end to relax this time.
Yes, we go home tired, but the memories are worth it.
