With my first the last trip we took by air (which was to Florida, though not Disney) was right before Christmas and my daughter was born on 2/26 (though due 3/12 if it makes any difference timewise for you). So just about 2 months before she was born.
My son was pretty much the same, we flew to visit my in-laws around Memorial Day and he was born August 4th. We did make several trips to NJ in June and July and a trip to Sesame Place where I went on several rides (though no water slides!) on July 17th. The thing is though, he was due 8/29 and was born at 36 weeks, so it wasn't like I was due 2 weeks after the Sesame Place trip. Sesame Place was about 2.5 hours away (by car) and NJ is 4-6 hours depending on traffic.
And about not thinking about preterm births with a normal pregnancy, both of my pregnancies were totally normal but my son was preterm. My water broke at 36 weeks with no apparent cause and I just had an ultrasound the week before to measure him (we suspected he was large) and everything looked fine. He was okay, but did have to stay in the hospital an extra day and then was thisclose to being readmitted once we took him home cause his jaundice was so bad as a result of being preterm and huge (he was 8 lbs. 4 oz. so his 36 week liver just couldn't handle his 40+ week size). So while I never thought I'd have him that early (in fact, I was thinking about another Sesame Place trip the weekend he was born!) cause my pregnancy was uneventful and my daughter was term, albeit 15 days early, I did, so with the next baby I'll definitely think twice befor going anywhere past about 32-34 weeks.