Hi Laura,
I was 5 months pregnant of my eldest (8 years ago!) when I went to the World. I remember that I was taking Lamaze clases and the instructor recommend me to rent a wheel chair. I didn't take her advice, I thought she was exagerating, and I was used to exercise so walking a lot didn't bother me.
Well, it was in May and the heat was intense and I stood a lot in the first couple of days, so at the end of the 3rd day I was definetely not OK and ended... yes, in a wheelchair.
So DH pushed me around for the rest of my vacations, and I was better then.
My advise would be: even if you're used to walk, if you feel tired after the first day or so, rent a wheelchair! It will be more comfortable for you.
Good luck!