I took my son for the first time last September about 3 weeks before his 2nd birthday. As PPs said - don't stress over it and take the trip at your own pace. Your daughter will let you know what she likes and doesn't.
September can be hot so I made sure I had sun screen, water/juice, a change of clothes and some of his favorite snacks with me when we were in the park.
He LOVED the play areas in the different parks. Pooh's Playful spot, Donald's Boat, Dinoland. It was great. The areas are pretty much enclosed with one entrance / exit so you can let your kid run around while keeping a watchful eye on them. They get to play with other kids their age and for an active child like my son it was a great way for him to let off energy.
I had a very basic itinerary for our trip. Simply what parks we planned to go on which days. I gave the most time to MK and none to EPCOT except for some dinners and Nemo ride. I also went to allears.net and reviewed the various menus so I knew which places would be best for my son. We were on the dining plan so the only other planning I did was make our dining reservations. I tried to pick restaurants that offered food that my son would like as well as a fun atmosphere for him.
Most of my planning was not in what to do each day, but what to bring and how to pack the stroller so I had what we needed and it was easy to lug on and off the busses!
As far as afternoon breaks - again, your daughter will be the judge. Some kids can sleep in their stroller. Some can go all day and then you head in a bit earlier. Some days my son took a nap in the afternoon and others he didn't. It will be a fun learning experience for all of you. Just familiarize yourself with the parks and different attractions and such and then go and have fun. We're going again this year in September and I'm really looking forward to it.