I'm from MN and I've only done one Marathon Weekend (2023) but I have done several January WDW visits and weather is a total gamble. It's nothing like FL summer of course, but you can still have extremely sticky mornings and you can have relatively dry mornings. No matter what it will feel much stickier than you probably get in CA, but it can range from not a big deal to being sweatier than in your life between two days. Mainly I recommend hydrating as much as you can, I also like drinking electrolytes in the days leading up and during the longer races as well like nuun or tailwind or gatorade, etc. Slowing down too, I find the humidity makes it harder for me to breathe.
For MW this year the 5K was the most sweaty I have gotten on a run in my entire life! I want to say it was 70-80 but it was like 90-100% humidity (the early race starts mean peak humidity) and within a mile I was already drenched. My race photos are so funny because I look like I ran a marathon, and all I did was a slow 5k.
Meanwhile the marathon a few days later was quite dry and much cooler, no heavy/sticky feeling at all even at 3AM and I hardly broke a sweat during the entire race! I'll take a sticky, gross 5k any day for 2023 marathon weather again!
I will say I find temperature is a much bigger indicator of misery in FL than the actual humidity %, I think it was pretty humid during the half but because it was so cold (45-50), I didn't really notice it at all other than the fog. I'd say the high humidity only starts getting problematic when it gets closer to high 60s and up. People smarter than me can talk all about adding the dew point + temperature to better measure that but all I know is that anything over 70 degrees by 5AM gets me very nervous in FL, no matter the humidity %