It doesn't matter how fit you are. When you have to race in 70 degrees and 90% humidity, you can't realistically aim for the same the same goal as you would in 50 degree dry weather.

Tomorrow's weather forecast is exactly like that. 67 F at the 7:00 start, 71 F at 10:00. Dew point is pretty uniformly 66 F, which corresponds to humidity going from 92% to 84%.

I'm pretty sure I have no chance of running 3 hours in this weather. I'm not sure I should even try. According every heat table, it is already a good performance if I run 3:10 tomorrow.