Even though I had run a half mile last week at a slow pace for the triathlon start up training, I went for my first real run yesterday (in 3+ years). It was directly after biking, so my legs were already a bit weak, but it was humbling. My lungs burned that cold burn when you know your cardio system is weak, and my heart rate was very high by the end. It was 13 minutes of ego bursting. It was good. No way am I going to prepare for this triathlon well without really pushing myself. After weight training the last 3 years, I have usually done 10-15 mins on an elliptical machine. I see now how illusory that cardio was. It was a means of keeping my heart rate up for an additional 10-15 mins, but did not build up much lung strength.
I still can't believe I used to run long sessions of 8-9 miles some weeks for x-country.