Did 25 miles yesterday with only 951 ft of elev. gain, temps in the low 90's but the humidity was tough. Have not felt breathing difficulty like that as long as I can remember. I ride in all the temps in an effort to acclimate but this year is a new ball game. I am looking to do my first century this October but I think I will have to settle for a Birthday ride and do 72 miles. At least that way I also cover the metric century.
If I didn't ride in the 90's here I would loose most of my summer. Just try to stay conscious of my hydration and stay on top of it. I am fortunate in that I can easily shift my rout and refill my water. This all sounds good until the humidity makes it feel like I am breathing through a pillow.
Frank.