Susan Magocsi, a vivacious 50-year-old Milton psychologist, loves to exercise. She’s training for a walking marathon in Alaska in June. She lifts weights. She does yoga and ballet.

In fact, by a number of measures — such as low cholesterol and blood pressure — she’s admirably fit.

Twenty years ago, the gurus at the American College of Sports Medicine told us to get off our duffs and get those lungs and hearts pumping. Eight years ago, they told us to pump iron, too.

Now, they’ve added a third cornerstone to their fitness guidelines — get flexible.

You head out the door, virtue personified, for the first run, or at least brisk walk, of the year.

You know — who doesn’t by now? — that regular exercise is great for your heart, your muscles, your mental health.