IT was interesting to read of Sir Peter Snell’s heart issues at the weekend. 
I remember reading an article years ago about the effects of high intensity activity on the body.
Essentially, it stated that some people were putting themselves at risk by continuously stressing their cardiovascular system in an attempt to improve their athletic performance.  While I can’t remember the exact detail, the excellent MacDoctor blog highlighted some more recent research in light of Snell’s news.
He writes: “I clearly recall a study done in South Africa 20 years ago in which the investigators took blood from Comrades marathon runners after the event (the Comrades marathon is a 90km ultra-marathon run between Durban and Pietermaritzburg every year).
Every participant in the small study had significantly elevated heart enzymes – the same ones we use to diagnose heart attacks. It is thought that this was caused by diffuse hypoxic damage (heart cells dying from lack of oxygen all over the heart)….(t)hat early South African study has been backed up many times. There is little doubt that athletes who train intensely are at increased risk of sudden cardiac death immediately after their training…(t)here is also some evidence that the increased amount of oxidative stress (free radical production) caused by intensive exercise does cause long-term damage to the arteries, resulting in additional plaque formation.”
However, he goes on to say:
“Moderate exercise is good for you. The body apparently easily copes with the additional oxidative stress, producing higher levels of anti-oxidants. These higher levels of anti-oxidants are then useful for mopping up the extra free radicals we produce from eating junk and being generally stressed out of our minds. At least, that’s the theory.
But competition-level athletes pay a price for their extreme conditioning, both in physical form, with injuries, and in physiological form with potential heart damage…the old adage is true – too much of a good thing is bad for you.”
Indeed.  When I was training flat out for athletics, our goal was to push through our recommended upper heart rate to gain some benefit.  After 40x400m, you knew you had a heart, and lungs.  The downside was the dizziness (which my coach and I dismissed as just hard work, because surely a fit 18 year old isn’t really stressing their heart).
The problem with high-intensity exercise, whether you’re training for the Olympic 1,500m or just pushing yourself to improve your middle-of-the-road 10km time, is that it’s addictive.  You feel so good afterwards that you want to repeat that feeling.
So you push a wee bit harder, and get that buzz.  We are a lot more aware today of the physiological effects of exercise, and so high level athletes are constantly monitored for potential problems. 
My high-level athletic career ended about the same time as Jim Fixx, the author of the Complete Book of Running, died of a heart attack on his daily run in Vermont.
To be fair, Fixx had some risk factors that contributed to his heart attack, including stress, having been a smoker, and a family history of heart disease.
So what’s my point?  Enjoy your sport.  Sure, push yourself, but not to the point where it starts to impact on your general health.  Listen to your body, and be aware of your limitations.
After all, us old fellas aren’t training for the Olympics, just for an excuse to have the odd beer or three and a plate of deep fried calamari without guilt…