I was looking for some kind of metric that I could use in 2011 to have some kind of idea improvement. After this discussion here I've decided that average speed just isn't it. I think an occasional self-imposed 'time trial' will get me closer to what I was looking for, along with general fitness/heart rate/how my body feels/etc. Thankfully my heart rate recovers quite fast, and while I certainly won't say that I never find myself panting, it passes quickly. This does help

I do appreciate all of the comments, they have been quite helpful