milestone?

Last Thursday, I think I may have hit a milestone.

I was running on a treadmill, and somewhere near the fifth mile, my legs started to give out, and my lungs were fine. My legs were actually so tired that they refused to move fast enough to keep up with the treadmill's motion, and for a split second I actually felt like I was going to fall off the machine. Yay for increased cardio fitness!

Although, I guess the alternative explanation is that my legs were having a bad day.