This trip we will arrive aroudn 11:40, but don't have a park planned for that day. We were just too tired last time to really enjoy DTD, so we are swimming and doing DTD the first day.
Last trip, we got to MK around 3ish. We met a friend for a few hours at donald's boat, then took the train back to the front of the park. Then the ferry to WL for Whispering Canyon. I was glad that we sat down for dinner (as I am most days). We went back to the park for 2 or 3 rides then left. It wasnt bad (kids were almost 3 and 5), but I am looking forward to a relaxing day this time.
I think that if you are not going to a park the second day (or it will be a leisurly park day), it is do-able.