Alternate with Hard and Easy for Maximum results

Lately I have been training with a lot of intensity one day and taking the next day almost completely off from training. Today I ran an easy 30 min jog as slow as possible since yesterday was a very hard workout for me. By alternating like this, I have had much more productive workouts on the hard days. Sometimes I forget that the real benefits occur when your body is resting and rebuilding from the previous workout. I have been focusing more on recovery and noticing huge improvements in my aerobic fitness. I have not doubt that in 1 month I will be able to run 20 minutes for 4 miles if I can continue doing a lot of intervals at 5 minute pace. When I was running 3:45 for 1500m, a 20 minute 4 minute tempo run would be a bench mark workout. My main focus this winter is going to be building aerobic fitness through threshold training.