It's interesting how differently people perceive pushing wheelchairs (difficulty level). Whenever I ask my spouse about this, he tells me that he doesn't find any of the pushing very difficult, even the steep ramps at the TTC. He is almost 50 and is certainly not a marathon runner and doesn't work out at all.
He has told me that the only problem he has ever had with the wheelchair is to stop it quickly enough so he doesn't hit someone when they come to a sudden stop in front of us. The last time we were at WDW, I picked up a pedometer for him (I was curious to see how much walking he did).
We were at the Swan, so he would push me to both Epcot and MGM (he doesn't like how slow the boats are), and most of the time he would push me through Epcot so that we could catch the monorail to the Magic Kingdom. The only time we took the bus was to Animal Kingdom.
He averaged about 12 miles a day and never had a single blister, sore muscles or any other problem. I think that how much problem a person has pushing a wheelchair just seems to depend(smile), because I can assure you, he works with computers for a living and really doesn't get very much exercise, except when we go to WDW.