Help me feel better, please

Any takers?

Well it definitely depends on what 20 to 30 degree increase you are talking about. If you go from 40f to 70f, I don't think it will hurt you 20 seconds per mile, no, maybe only a few seconds per mile. But if you go from 80 to 110f, it's going to really set you back 30+ seconds per mile.

Heat index is a good way to look at it. Heat index below about 60 and you'll be feeling great on distance runs. Heat index from 60 to 100 will probably hurt you linearly. Heat index above 100 starts to hurt you exponentially.

Will your marathon be in the same conditions? Your workouts will be slower in the heat and humidity, but does not mean you get less of a benefit from the workout.

We’re talking about temps in the 40’s and humidity in the 70’s when I trained in the fall a d temps in the low to mid 70’s and humidity 80-100% in the last month.

Heat index low to mid-70’s.

Marathon should be low to mid-50’s and 70-80% humidity.