I went and checked out Mr. Burney's Blog, interestingly he's got a story about someone who just took up cross and the adaptation he had to make from a very high level of fitness. This fellow's problem was that he had a hard time hammering back up to speed out of corners and other slow bits. The constant accelerations of cyclocross are unlike almost every other cardio-sport. More like an hour of BMX intervals.
That's probably the most common issue for people new to the sport. Humans tend toward efficiency. Unlike most endurance sports competing in cross involves wasting a lot of energy in lots of bursts.
Anyway, have fun it's a great sport.
Ron