It takes years to build up endurance. It's always rather amusing when the mags or web sites say that "RC trained really hard for the two weeks between these nationals and it paid off." In two weeks or even two months you can mostly do damage.
I have done semi-serious endurance training (and I started from a defensive lineman's level of fitness), which means about 3-5 hours of effective training weekly (including riding, which is about 1 hour in the winter and 3 in the summer), and can ride full speed for maybe 5 minutes(I still have enough muscle to force the 450 around, and that tires you out fast), and nearly full speed for about 25. I can still make the jumps and ride at a reasonable pace for 35 minutes.
If you start endurance training, find out your heart rate thresholds and present fitness level. Endurance training is surprisingly "easy" when you are building up basic endurance.