Originally Posted by Racer Ex

Anaerobic stuff is the most trainable in the shortest time period.

40k's are different beasts than 20k's. They require a lot more technical expertise and discipline. The penalty for a lack of either one of those is pretty big. If that's your goal I'd ride as many of them as you can, and draw out a 40k course you can practice on. I'd want to ride that 1/2 times a month; set marks at 10,20, and 30k and note the time you arrive at each point. Then go back and figure out where you can improve.

In 95% of the files I see people go out too hard.