The atmospheric humidity is a factor very determinant when it comes to workouts, feel the discomfort of excessive sweating and of course fast dehydration, is something that affects the performance of any. But with proper training the body adapts gradually!
Success in your races! keep going on!
I usually bring a small face / sweat towel to mop my brow when sweat builds up (and it doesn't evaporate at these humidity levels). I haven't needed it until this week but forgot...noted. Today it hit 90 degrees F and it's been hot for this time of year.
Hopefully I adapt to the heat a bit, it just takes awhile each year to get used to it again. My ideal condition is about 30 degrees F cooler than it was this week (low 50's F / ~10-12 deg C).