Touring Plans gives the park crowds a 1 out of 10 for the first few days in September when we are there - if that turns out to be the case, that would be brilliant. Even on Labor Day weekend - they are talking low numbers - surely that cant be right, especially with the Half Marathon going on - our first full day is the Sunday 31st - day before Labor Day and had planned to skip it and go in an extra day on the Thursday (departure from Disney day) but Sunday is a 3.2 at DLR and a 3.9 at DCA - what do you think???
Wow that would be great! But I wouldn't hold my breath. With a holiday weekend and RunDisney event, I'm expecting pretty big crowds. Having said that, the running crowd will tour a little differently to general park-goers, which might work in your favour.
I'd say most runners will be leaving the parks early, either in preparation for a super early morning (we're aiming to get to the corrals by 4:30am

), or just in exhaustion AFTER a super early morning followed by a good workout

I suppose on Sunday you're probably more likely to get people staying out later since there is no run on Monday morning, but other people are likely to be going home after the run, so that might even out.
I can't work out whether there will be more or less people in the parks in the early morning though. The runners and their families will already be up, but some may choose to take it easy for a couple of hours after the run, go back to the hotel and freshen up, even have a nap (I know that's what my dad is hoping for!).
I arrive on the Tuesday before Labor Day, and we leave on the Sunday (only doing the 5k and 10k, not the half marathon). My plan is to hit the parks til closing on Tuesday, not go at all on Wednesday, go again on Thursday after the expo, but leaving by 8pm. On Friday we plan to hit DCA at opening for TSM and RSR FPs, leave for a rest, then go in again at our FP return time (hopefully late afternoon) and hit a few more rides while we're there, leaving by 8pm. Saturday we plan to go to DL at about 11am, in time to get a FP for Indy on our way through to our BB lunch ressie, and stay for as long as we can.