Marathon effort, not pace, during hot weather?

With this heat right now, I would not worry about your pacing.
I would shorter training sessions which reflect the weather you are facing.

fair enough; I'm trying to build base but may just focus on tons of EZ volume to get that up and not do speedwork.

What’s the dew point? 90 in Sacramento is different than 90 in Miami.

Dew point is 70 in CLE.

This is what I always did, and it never hurt me on race day. I've done marathon pace workouts in hot weather about 20-30 seconds slower than goal pace. No issue hitting and holding goal pace on race day in better weather.

Split some of your runs.

30 seconds is about how much I slow down...but it can be even more if it is particularly humid

Yes, assuming your race will be cool with low humidity, your solution makes perfect sense. If you draw an unusually hot race-day, then you might have adjust your goal time and stay closer to your heat-adjusted pace on race day.

According to the advanced features of this calculator, the temperature effects are:

20.4 seconds per mile; 12.7 seconds per km

When? Call me when it’s 90/75. 90/70 sucks but isn’t miserable. Especially if you’re not in direct sun. It eclipsed 70 two days in the middle of the day. Why are you running at 3p? In full sun? When it’s in the mid 90’s? When the dewpoint is over 70? It was 2 days. Just 2 days of awful weather. But it was fine in the morning and evening. You need to toughen up. It hasn’t even hit summer yet.

Dumb question

I ran at 11:40 a.m. so part of that is on me for not running earlier, I admit.

The summers have been crappier the last few years, IMO, I've noticed. We haven't had a cool summer since 2000 (in CLE).

Yes, absolutely! I recommend you use a heart rate monitor strap to help you dial-in that effort, too!