Improvement?

So in a weird change of events, I improved my mile running time by 59 seconds tonight. And, when the mile was over, I could have kept going for a bit longer I think. My legs weren't feeling like they were going to break and my lungs weren't collapsing. Yes, I ran the mile in 13minutes and 6 seconds. Which is still not ideal, I realize, but major improvement. Could this mean that I'm actually building my running capacity already? I hope it's not a fluke. I walked another mile and was so happy I threw in a couple running intervals (at a much faster speed than my mile) just for fun. Side note- I think my ipod selection might have helped. I used RPM music, which is pretty fast beats per minute. So I think that might have helped me set a rhythm to my pace.

Monday night I walked for 20 minutes and then taught my spin class. And last night, another possible improvement-I swam my 40 laps, 10 laps and then some rest, but the last 20 laps I swam without stopping. Which was extra surprising b/c I thought I was having an off night. I left work late and was not feeling swimming at all but I made myself. AND I could do 20 laps without hanging onto the wall or stopping.
Is my overall fitness level increasing? Could it be? I think so unless it's a coincidence. So, my question is then- why haven't I lost a damn pound?! And don't give me the "you're building muscle" speech. I was a personal trainer and I've used that one. And it's sometimes true, except when you're already sporting a bunch of muscle under your fat-tasticness, which I think I am.