You could play with doing 1 workout a week with ~10k in volume @ T instead of 2 workouts with around 5k volume. Specificity is important in half marathon/marathon training. 3 x mile or 12 x 400 are fine but 5k pace isn't going to get you to your best half marathon. Turn those into 6 x mile @ T w./ short rest (1:00) and 20-24 x 400 @ 10k (slower to start is fine) w./ short rest (45s). Stuff like that.
I will definitely try this out. As much as I felt I was getting more fit with the quality work I was doing, a lot of it felt "short".
I'll also add that many people struggle to race as it warms up coming off of winter training. So even though those conditions aren't that oppressive, they could be relative to what you're used to. I'll leave it up to you if it applies.
This is something I thought about. I'm in Florida, been running in mostly beautiful weather for 2 months, then 3 days before the race we got hit with the warmth and humidity. The big workout I did 6 weeks out from the race was run in mid 40s temp. No wonder it felt so good.
Raced another half yesterday and it went considerably better. 1:35:16.
Weather was more favorable at 60 degrees, and I paced very conservatively. Went out the first 5k in 24ish. Middle 7 miles worked down from around 7:30 to 7 flat which felt very comfortable, it didn't feel "hard". Closed the final 5k in 20:45.