Do you have a trainer? My fluid trainer came with a Chris Carmichael mountain biking dvd. Lots of max effort intervals, so not really aerobic (more anaerobic because you are at max effort). Wow! It is has really helped my stamina immensely! I switch out my back nubby (knobby?) tire with a sturdy, slick (26"x1") tire when using the trainer because a) the nubby tire makes a lot of noise, and b) the trainer wears the tires out.

Here is a link to the dvd at REI, but I bet you could get it through amazon or something:

http://www.rei.com/product/726223