Recent studies reported at the Mayo Clinic show that endurance exercise especially done o the extreme as in marathons, half marathons, iron man, etc...can cause damage due to stress in the heart. This damage gets worse when the entrance exercises are repeated.
So if you love to run, and run miles and miles, or bike mile and miles,or swim miles...you he the idea..you may be hurting yourself.
Regular non extreme cardio an be a good thing, but like anything taken to extremes it can be dangerous.
Strength training with flexibility stretching and some added cardio if u must...is a better plan!
Report on endurance cardio follows...