As others already say , two are two different things . I like run in heat and my pb in 10k and shorter race are made in the summer with 30° celsius. To train in the heat is an art, of course we need to choose early morning (better) or late in the evening for quality workout. To avoid absolutely the hottest hours, cos it can be dangerous, except maybe for some easy run.
Yesterday at 7 in the evening I did a 6x1000 rec 2' and after the 3rd repetitions my body temperature was very hot , no singlet of course , and maximum sweat productions, it was under trees but 33° celsius. Now here are the first days of heat
so next time I know that the body will adapt and i will feel more "confortable" running at Vo2 max / LT in that heat.
For the same reason another workout that is harder is the long run. doing 30 KM with 30° is all another story that do it with 15°. So for me no need to push the body to do long run more than 2 hours with extreme heat condition, I usually stay on 20
25 km and then I know when temperature goes down a little it will be easy to do a 30km or more.
About the speed I must say that I found it easier to make speed workout in the summer heat than in cold winter, is like my muscles works better.
Anyway in the summer heat pushing too hard is not a good idea, we need to recover well every workout, is more easier to go in overtraining, or chronic fatigue state, it happens easily in the summer, maybe in a month I do 3 races with good results than the next month all in a once i feel with zero energy. Stop , finish so I need good recover to be again in good shape