I was trying to do the whole ride, ride, ride thing to get a boost in my stamina prior to a trip to Brown County, and it was working pretty well, but the biggest improvement I saw was a result of a single piece of advice from a riding buddy. We were out riding and he noticed me getting winded on some of the tougher climbs, he told me to really focus on slowing my breathing down and breathing deeper while climbing and recovering. Bam, it was like someone flipped a switch. All of a sudden my legs didn't feel like lead halfway up a hill and I was recovering MUCH faster.
I guess the point I'm trying to make is that making an effort to improve your... I don't know... "aerobic technique", maybe?? whatever you want to call it... can have as big of an impact as just getting in the miles.