I second just about everything said here so far. I started travelling alone with my DSis when she was 5yo (I was 25yo). Now, since DH won't go with us, I travel alone with DD (almost 3yo), though now 10yo DSis sometimes still comes. I rented a stroller for DSis the first 2 years just to make sure she didn't wear out too quickly. For DD I either take our stroller or rent.
When DSis wanted to swim, we would sometimes go back midday. With DD, who doesn't swim yet, we don't take midday breaks very often. She falls asleep in the stoller easily so I just hang out and people watch, take in a street performance, or have a leisurely meal while she sleeps. When she's rested again we pick up where we left off.
Planning wise, I always plan which parks we are going to each morning and evening. I also tend to make one ADR per day as if give a nice relaxing time away from the bustle of the park. From there, I let DD decide what she wants to do. I also make sure I know the best way to beat the lines on the things that I know she will want to do (e.g. hit Fantasyland early, etc).
Have fun!