IT'S ALL VERY well for Oscar H. Franco of the Erasmus M.C. University Medical Center in Rotterdam to tell us to exercise more. But when does he think we have time for this? As Rob Stein reported in The Post yesterday, Dr. Franco's study in the Archives of Internal Medicine suggests that if people engage in some low-intensity activity such as walking for half an hour a day, five days a week, they extend their lives by 1.3 to 1.5 years; if they spend that time in a high-intensity activity such as running, they can expect to live an extra 3.5 to 3.7 years. But what about the time spent doing all that exercise? And showering afterward? Do the gains justify the investment?

The walking option appears to be an easy win. If you walk somewhere rather than taking the bus, the trip may take you twice as long -- say, 30 minutes rather than 15 minutes. That extra 15 minutes, five times a week over a span of 12 years (the period Dr. Franco used), adds up to an investment of 780 hours. One should convert those hours into days at a rate of 16 hours per day, because you need to sleep the other eight or else more medical researchers will come after you. So this means the walking option costs you about 49 days of your life. If the benefit is 1.3 to 1.5 years of life expectancy, you've got yourself a bargain.

Now, what about that running option? Most people aren't going to run to the supermarket or work; this is not an alternative to a bus trip. Intense physical exercise may require getting to the gym plus time afterward to shower; so, whereas 30 minutes of walking may cost 15 minutes of your time, 30 minutes of the intensive stuff will cost you, say, one hour. At five days a week for 12 years, that means 3,120 hours, which works out to the equivalent of 195 days. If Dr. Franco is right, this investment buys you an extra 3.5 to 3.7 years of life. Again, a clear bargain.

Yes, yes, we know; you still have some excuses. Time today is worth more than longevity some decades hence: You could be run over by a truck; the future is uncertain. But if you discount the future at a rate of 3 percent a year (a rate suggested to us by Robert W. Hahn of the American Enterprise Institute), exercise still looks like a bargain. For somebody in his mid-forties, there's a 30-year lapse between the exercise and the time gained at the end of life, so that gain should be reduced by a factor of 1.03{+3}{+0}, or 2.4. So an investment of 0.5 years in intensive exercise buys you about 1.5 years of discounted future life. Sorry folks, that's still a bargain.