There's a discussion in 'The Art of Learning' by Josh Waitzkin about interval training. The book is really really good, so it's worth a read anyway, but the short version of interval training is that you train yourself to max out, then rest. You perform for longer periods at higher levels and rest for shorter and shorter periods.
I think Daven mentioned this earlier in the thread. When you're tapped, you're tapped, but if you can take a 2 minute break and come right back at 100%, you should be able to get much better results. (Your overall results aren't that bad anyway)
