Fifty is a great age. We are often at the top of our game with careers and families. Everything is right where we want it to be.

From a health perspective, though, we can start middle-of-the-road. Just as we’re feeling accomplished and settled in other areas of our lives, the colonoscopies, annual mammograms, and prostate exams remind us that we’re not as young as we used to be. No matter how vital you feel, getting anally probed definitely isn’t something that you associate with youth and vitality.

Right around 50 another thing generally starts to happen. We start to lay off on the vigorous exercise. Ask a bunch of 20 year olds what they do for exercise and you’ll get as many answers: sports, gym, running, swimming. Ask a bunch of 60 year olds and you mostly get one answer: walking. Somehow when we turn 50 we forget about vigorous exercise and slow down to a more gentle pace.

It turns out that this decision to hang up our track shoes in favor of walking shoes plays an important role in how long and how well we live.

There’s no better illustration of this than the Harvard Alumni Health Study on cardiovascular health. Looking at more than 17,000 people followed over fifteen years, this study looked at how the intensity of exercise impacted the risk of dying from cardiovascular disease.

The average participant was about 57 years old and burned an average of 2,000 calories per week. Their exercise was divided up as light, moderate or vigorous. The interesting thing is that it didn’t really matter how much time the participants spent on light or moderate activities – there was little association with cardiovascular outcomes. Only those who participated in vigorous activities saw a decline in their risk of heart attack or stroke, and that risk got less the more they did.

So what qualifies as vigorous activity? Running, fast cycling, stair climbing, or shoveling snow qualify. Light and moderate activities included walking, gardening and golf. The lesson here is that if you want to live longer, don’t walk for exercise. If you do want to walk, consider walking to the gym for a vigorous workout. Or walk to dinner after you’ve finished your cross training. Just don’t walk for exercise.