[throwaway848483]

It may seem stupid, but putting an extra tee-shirt helps a lot. Even without feeling cold, that extra bit of warmth remove a stressor which can tip the balance.

Slowing down a little helps, and erring towards the more comfortable.

The idea behind, is that if you are pushing to the max during summer when the conditions are favorable, then when the conditions are less favorable, you should allow some slack and not expect to be at the same levels, otherwise you will be burning out trying to compensate. (A car analogy : you wouldn't try to beat your lap time record on a rainy day).