Heat wave running

I am in my peak mileage week of marathon training and I have a 20 mile run (which will be very difficult for me) planned for Saturday, but the low is 80 degrees….so even if I set out at 5AM, it will be super humid and almost 90 by the time I finish.

Is it worth pushing to Tuesday when temperatures are more back to normal? Or should I just try to power through it to stay on my weekly schedule and not mess up my taper? So far, temperatures have been manageable for my 15+ mile long runs.