I'm coming back from quadruple bypass surgery and trying to reach my optimum level of fitness for my age (66.) That means pushing myself, but doing so with caution. Forty years ago I was a serious endurance athlete - mainly long-distance x-country ski racing and running. Many of the men I competed with are now struggling with atrial fibrillation.

Here's the problem with AF: the most common outcome is not a heart attack but a stroke. I had a single episode of AF about 1 years post surgery a couple of day AFTER pushing myself past my limits. Two days after that, I had what is called a transient ischemic event or mini-stroke. I woke up in the middle of night and the entire right side of my body was weak. It only lasted a couple of hours - a tiny clot from my heart went to the brain and then dissolved, leaving no after effects. I don't know about anyone else, but I'd much rather have a heart attack than a stroke, which could leave me permanently disabled for the rest of my life.

Now, two years later I've had no AF. I'm on a very low dose of beta blockers and a full dose of daily aspirin (not a bad idea for almost any older person.) What was "over my limit" two years ago is now fairly comfortable and I wear a heart rate monitor at all times to gauge my effort - between cadence, heart rate and breathing I know exactly what I can do - the speed is what it is!

I've reviewed the medical literature carefully and have come to the conclusion that for up to 6 hours/week of moderate aerobic exercise you are contributing to your health. Over 6 hours you may be improving performance, but are slowly entering the area of risk. Just don't fool yourself: if you are doing more than 6 hours/week you are not doing it for your health but for other reasons. Those may be perfectly valid and reasonable - achieve personal goals, winning a race etc. - but you are not healthier than you would have been sticking to a more moderate plan.