Don't understand

I haven't run for almost 3 months and went on Tuesday for the 1st time expecting it to be slow and painful. Bizarrely I ran almost a minute and half faster per mile than I did the last time I ran.

I was really stiff afterwards, understandably. I went again today and again I'm much faster than I was 3 months ago. Not only that but it felt almost effortless today and I reckon I could get faster yet.

What is going on ? I've done no running at all in that time apart from 10 mins on a treadmill about 6 weeks ago. I have done a little cycling but only about once per fortnight.

I could hardly be accused of overtraining 3 months ago as I used to do about 10 miles per week. Has this happened to anyone else ? Maybe I just got into a rut of just plodding and the break has allowed me to forget it.