Encouraging signs

I went for a run tonight - freezing cold and very dark. I decided to try something different. I ran around the town on my usual route which is about 4 miles long and did it in 29 minutes but when I got home I just kept running and repeated the circuit. Absolute torture! Running past your front door and just keeping going is murder... but I'm sure its also very character building. The second circuit I did in 31 minutes which I was very pleased with.
A strange thing happened though. Half way round my second circuit I just came to a stand still. I felt exhausted and out of energy. I was seriously worried that here I am 2 weeks before a marathon unable to run 6 miles without stopping. However, I reminded myself that I'm still fighting this virus thing a wee bit and so didn't panic. I started running again and pretty soon found myself getting into quite a strong rythm and in fact I began to run stronger and stronger until by the final big hill up to my house I was able to run straight up with no problems at all. If I hadn't stopped my second half would have been faster than my first. Great.

Do any of you have any idea what just happened tonight?