Should be fine... I did FP+ a little further ahead than that but ended up changing a whole bunch of them while we were there anyway.
I didn't have any ADRs (have 3 little kids -- can't justify paying a ton of money on overpriced food that they won't even eat all of, so just QS and brought in snacks, PB&J, veggies etc) but my mom really wanted to go to Rose & Crown Pub, so she just called the dining phone number while we were there and got a reservation for later the same day. I'm more of a "general planner" than a "down to the minute" planner so it wasn't a big deal.
This was Thanksgiving 2015 and it was quite busy (actually I think it was the most crowded time I've been there aside from Spring Break 1998), and we had no issues. The only thing I never got a FP for was Mine Train, but only 1 of my kids was big enough to get on it anyway. As long as you relax and let the chips fall you'll have fun, try not to freak out!