My brevet riding friends and I almost DNF'd a 200 because the temperature in NJ was waaaaaaay lower than in NYC (and lower than predicted). The problem was that most of the first ~15 miles of the ride was behind a big ridge that blocked the sun in the early AM. At ~15 miles, the entire field, it appeared, stopped to warm-up in the town of Piermont. Brr! My friends and I eventually finished with a respectable time.
BTW, I'd like to echo the sentiments of a poster above. Being a fast cyclist with a fast bike makes the brevets a lot easier. When your biggest concern is "has anybody passed us?", it's a lot easier.
Well, true for 200k rides at least.