OP- I hope you still had a fantastic trip overall!
We've done two WDW trips so far in 2017. One in February/March and one in May/June. It's just so much more enjoyable in the cooler months. After our last trip, we decided that from now on we're only taking trips to WDW in the winter. Someone mentioned that September can feel worse than mid summer even though the temps are lower in September. Totally agree. Last year we went in July during a week where the high hit 100 almost every day. We went again in September when the highs were in the 80's. September was more uncomfortable because of the oppressive humidity. I'm in good shape and on the low end of recommended weight for my height, so I disagree with the idea that only overweight, sedentary people are affected by heat. Sure those things likely make it worse but just glance around on a hot day. You can tell that almost everyone of every age, size, and shape is feeling the heat.