Did I cook myself on the first "hot" day of the year?

Are you dehydrated?

The answer is "yes." OP needs to chug water until they feel bloated, recover, and chug some more.

Same thing actually happened to me... went for my normal midday run at 85 F, felt terrible most of the run w/ stopping, one mile away started to feel weak/lightheaded and needed to jog it in at 9:30 pace. Was definitely "dehydrated" in the 85-degrees context so I spent the rest of the day fueling/hydrating.

Took me about 2 days to come back, the next day I was sore in some weird places.

Yes, you are well done now and done for, so know here on that that’s not done, and that undone can be done but well done can’t be undone.

I wonder if you might have also messed up your electrolyte balance? In addition to water, I'd add some electrolytes or at least some extra salt today. Lots of water doesn't correct the loss of electrolytes.

Don't run for 2 days and take some walks instead. Do what the others here have posted - drink water, get electrolytes, eat healthy, and get your sleep. After those 2 days off, you'll be fine and itching to run. That first really nice day is very tempting to run too far and/or too fast. Have had it happen a number of times over the years to myself and those I coach. Trust me, this works.

The good news is if you are alive today you are going to be ok. Your body does not allow yourself to do serious damage, especially on something like a training run.

You found out the hard way that your body isn't acclimated to heat. Wait until the heat returns and keep training. It will take you approximately 1 week to get used to it, regardless of your level of exertion.

Had a couple of easy days, slamming electrolytes and fluids and getting as much sleep as possible. Felt normal but HR is still a smidge high on runs.

Did a session today, just went by HR and its almost back to normal but maybe missing that top 1%, threshold HR was 1-2 seconds per km slower than a few weeks ago - but maybe that's down to other factors too.

First track race of the year this weekend so hopefully I'll be ok by then