Warmer weather and longer days helps to increase the average speed. I'm not doing anything like Steve's mileage but I've been doing roughly the same amount each month and in the last couple of rides I've felt my speed, power, and general well-being improving. As its not down to the training I put it down to climate.
(And the same thing happens in reverse in November, when some b*&*er goes round and makes all the hills steeper)
All the same I'm still glad that Steve is proving his theory correct - that he will speed up as the weather improves. And at 180 miles before it gets dark, all I can repeat is:
Go Steve!!!!