For the alternative view....
I push my DW in a wheelchair when at wdw. She's probably a similar weight to your DH, so, I know that it is possible. We've considered the
ECV, but, for us, the w/c is best. There's a certain trust/intimacy that comes from it that we enjoy, plus a greater flexibility to fold it up when there are others needing the tie-downs on the wdw buses.
(Btw, we have learned that the best option is to rent from home since this both makes it cheaper and means that she doesn't use up her walking power in airports.)
Now, to agree with everylastbreath....
Does pushing someone up and down hills, over the themed AK pavement, etc. wear me out? Absolutely. What do you mean when you call yourself "in fairly good shape"? The first time I pushed my DW at wdw, I was trained for a half-marathon and I found it challenging and tiring. After that I started lifting weights and pushed myself to improve my upper body strength--I would still not have qualified as a serious lifter, but, I roughly doubled my arm strength and it made a big difference. This year, I've had surgery twice and my weights are down below where I started, I tried pushing my DW last week in DC and was easily exhausted. Disney is a lot of walking. Can you walk 15 miles per day everyday without being too tired to enjoy it? Now, can you add a 50lb pack to your back? My experience is that this is a comparable comparison to the workload.
As for your DH walking, I don't recommend it. Broken feet can be really challenging healing experiences. I have known many people who walked too much in their walking casts and ended up in significant pain and/or with chronic problems. The easiest way to keep him from overdoing it might be to rent an offsite ECV.
My DFIL (with the same condition as my DW) came with us in Jan and we talked him into an ECV. He would use it for the room to bus, bus to park, and land to land walking. Then, he frequently parked the ECV with the strollers and walked around the local area. He remarked several times on how much more he was able to do because he didn't use up his foot power on the boring parts of the day.