As a first timer, you're probably going to want to try nearly everything at least once. I'm going to assume you have kids.
2 for magic Kingdom
2 for Epcot
1 for MGM
1 for AK.
1 extra in case things go awry and you just don't cover one like you wanted to or to spend an extra day in the park of your choice.
Total of 7.
On our first trip we did
2 in mk. We covered everything with some time to spare.
2 in Epcot. We covered everything but we didn't do all that much looking in the shops and stuff in Germany, China, Italy, or Japan. And we skipped the American show. Toward the end, I was carrying the kids and hurrying to get to every country so they could get their rewards (which were goofy prints.) When he got em all, the CM got up and made a big announcement and everyone clapped. But if you don't linger too long in the shops you can cover it.
1 in MGM. We had to make choices here. The show times don't allign all that well and you'll have to choose.
1 in AK. If you skip Rafiki's planet watch, you can cover all the rest of this one fairly easily. We missed a show and didn't have all that much time to enjoy Dinoland. Rafiki's was good. They were doing surgery on Timon (Ok it was just a meerkat.) But it's very time consuming and you don't realize you're consuming it. 5 minutes to wait for the train, 5 minutes to ride the train, 15 minutes to go to all the exhibits and see demonstrations on the walking trail to the building. 5 minutes to grab a snack at a diabolically well positioned snack cart. About an hour at the building (including watching the surgery.) 10 minutes at the petting zoo, 5 minutes to walk back to the train, 5 minutes to wait for it, 5 minutes to ride it back. We 2 hours there and didn't realize it. We missed a couple things we really wanted to do because of it. And note they don't always have a cool surgery to do. So you may get there and they're doing nothing.
1 at Typhoon Lagoon
1 doing nothing. We went to the parks 4 straight days. We found we needed a break. When we went, we had a nice couple next to us that had been there 4 days before we got there. He said about after the 3rd day of tromping around, your legs just get numb. We joked about it all stay. So do take your rest intervals.
We had a 7th day in the parks on our ticket. I would suggest you do that too. We didn't quite cover AK or MGM, but we elected to go to the MK again and catch what we missed the next time. They weren't really worth an extra day and it really wasn't worth the price to add hopping. to finish up parts of both that we missed. Fantasmic almost got rained out when we went. Had it been rained out, we'd probably have gone to MGM that last day to catch it instead of the MK.
The parks are big. Epcot is huge. Tons of space between buildings and rides. The park maps do not do it justice. AK is even bigger, but much of that is taken by the Safari (which you ride in a vehicle) and the Planet watch (which is partial walking, partial train, plus the animal habitats when they bring em in each evening you don't see. So imo, there wasn't quite as much walking to do there as at Epcot.
MGM isn't too big. It's a long way from the Fantasmic end to the Stunt show end, but most of the park is fairly compact relative to the others imo.
As far as walking goes, here's my advice. Start walking now. It's a ton more than you think. About a month before you go, buy two new pairs of comfortable walking or running shoes. Don't by the cheap
Walmart shoes. Get some good ones. Make sure the heel does not slip. Break each in for about 1 or two walks. Then put them away until your trip. Even if you don't get the shoes wet, alternate them each day. Give the other pair a chance to air out. After your trip, you can wear them as normal. But I always alternate mine now. I was surprised at how much better my feet feel all the time just alternating pairs of shoes.
It will save you blisters, it will save you pain. Carry blister pads or moleskin, just in case. We didn't need it.
Thanks. I be there the end of August. 
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