Speedy gonzales??

Today was my long awaited BONUS day off work (bonus meaning it wasn't leave or a weekend and I just got to do what I wanted all day). The day looked pretty horrible- windy, cold and rainy, so decided to opt for the gym plan, rather than the long bike ride (fingers crossed for tomorrow). I slept in, had breakfast and then headed to the gym, where I did a relatively hard spin class (well, it was called "Challenge") and then rode for awhile on a horrible stationary bike (yuck yuck YUCK) before I got completely bored (only about 25 minutes). Decided to do a 1m run off, so hopped on the treadmill, thinking I'd hopefully do about 7:40/7:50. Started on 13.0km/h (a bit over 8mph) and gradually increased the speed, as I got bored. Ended up hitting the treadmill's limits at about 6 minutes- I'd chosen an old one (which I now know top out at 16km/h/10mph) rather than the good ones (which go up to 20km/h/12.5mph). Anyway, the short version is, I hit the mile in 6:40! AND when I got off, my HR was 150??!? AND I know I could go faster, but for the dumb machine AND I definitely could have gone further. So, I don't know if I'm speaking too soon, or if it's just a function of doing an afternoon workout on a good night's sleep, or what, but maybe, just maybe my fitness is coming back?