This question comes from a man in his thirties.

Before starting any fitness or exercise program, be sure to check with a doctor if there are any questions about your health or capabilities.

First, let's look at the guidelines put out by the US Government. There is a helpful website available at http://pag.airhealthprojects.org/paguidelines/adultguide/default.aspx, and a PDF of the information there can be found at http://pag.airhealthprojects.org/paguidelines/pdf/adultguide.pdf

The 2008 recommendation is for 2 hours and thirty minutes of aerobic exercise a week, along with strength training twice a week.  This would work out to five sessions a week of aerobic exercise at 30 minutes per session.

How do you measure aerobic exercise? Commonly, people use their heart rates to check whether they are in the aerobic "zone" for exercise. Authorities believe that training at 60-70% of your target heart rate will improve endurance, and will build a stronger cardiac muscle. To calculate your target heart rate, just subract your age from 220. So for a 36 year-old man, max heart rate would be 184. 60% of this would be 110 and 70% is 129, so the ideal heart rate for maximum aerobic benefit in a 36 year-old man would be 110-129.

This is why you see people checking their heart rates in gyms, and why many treadmills and elliptical machines have heart rate monitors built in.

I think some healthy messages bear repeating here: some exercise is better than none, so don't feel like you have to stick to a time limit or heart rate limit religiously. Start slowly and track and build up your performance to avoid injury, especially if you haven't been active in a long time.

What has worked the most for me is to incorporate a little friendly competition into exercise--tennis has got to be more fun than a treadmill or stationary bike. I personally enjoy playing racquetball weekly, and I love going sailing on weekends I'm not on call.

Take care,
InteractMD.com