Two things: my wife was 4 months pregnant on our first trip and she hated it. She didn't walk a lot prior to the trip, though, so you be better off than her. You'll be at a point where any morning sickness should be gone, which is also good (my wife was still getting sick a lot, even though it was into the second trimester).
Second point, which no one brings up when talking about an infant at Disney - maybe I was just a worrier, but I didn't even like brining my infant to tag sales in the hot sun. There is no sun screen that a baby can use, and their skin is so fare, it just terrified me. I couldn't imagine having an infant at Disney. I don't know how I would do it. That said, you've got a lot more experience than me with infants (our only one is just 4 months now), so you should trust your own judgement on it, but I thought it was a point worth mentioning.
