I tried running on Saturday. ‘Tried’ being the operative word. Only a week and a half before, I walked home from work in the snow, but on Saturday it was 21 degrees C. I expected to do 8-10k but I got to 5 and gave up, shortly thereafter collapsing in a mass of sweat on the living room floor.
I did another 5 on Sunday and it was much easier in that according to Strava I did literally exactly the same pace and didn’t feel like I was dying. So apparently you can acclimatise to it pretty quickly, but it’s not instant.