Many people are fascinated with high intensity workouts. There are DVD sets available with 60 to 90 minute programs. Evidence shows that to much of a good thing can be bad for you in this case.
Too much high intensity exercise can inflame the tissues to the point where you damage the heart and cardio system.
Interval training works better and will be easier on the body especially of us over 50.
Many people get on a running machine, cycle or elliptical and stay at a pace for 30 minutes or longer. They may actually be damaging their system and seeing little progress in their health.
Running a marathon may not be heart healthy.
Use varied intensity for best results.
Runners- Try a warm up jog, then sprint for 30 seconds. Walk a bit, then sprint again. Repeat.
This is probably better for you overall than a long run at one constant speed.
This can be done on an elliptical too, where you may run at a modest rate and then speed up to get your heart rate up to near maximum for only 30 seconds. Try 30 slow and 30 fast. Your recovery will get better and your progress will improve.
A link I posted before also speaks to this---