We had a heat wave last week (max 39Β°C) and I did two easy runs (evening and morning run). I don't think I would've been able to do a speed session in that kind of heat. In Belgium, they even didn't recommend doing sports between 10 am and 8 pm. I can't give any professional advice at all but I personally believe that heat acclimation depends on each individual. I feel like there are definitely people who just just can't adjust to any heat. I do love summer runs, but anything below 30Β°C please haha. And summer evening runs are so cosyyyyyy! Just remember that it's okay to adjust your pace to how you're feeling. Better safe than sorry. I'm pretty sure the progress will be there!