those guys that do xc races in south africa and even hotter places must be incredible…absolutly incredible level of fitness to maintain it in the summer months in the day..
Your body acclimatises to heat, but it takes around ten days of daily exercise in hot conditions to do it. You adapt better to losing heat etc, which is why athletes do hot weather training camps before major events in hot countries.
So it might be that. Fingers crossed that 24/12 is dry and warm and breezy, but not roastingly hot.
[or it could be something else, I had a really bad spell a few years back and it turned out I was low on potassium, probably because in hot weather I was sweating out more electrolytes and not replacing them adequately, ended up using a potassium supplement, but I only knew that because I ended up having blood tests. Try using an electrolyte replacement drink as well maybe, Nuun or similar works for me rather than one with carbohydrate as well. But also, riding once a week isn’t a lot.]