Reese Witherspoon, 44, has done dance, strength training, stretching, and yoga for years. She gets up at 5:30 a.m. and hits the gym by 7:30 a.m. "I probably do that six days a week," she says.

You can't be as fit and healthy as she is in middle age unless you push it when you're younger — and then keep it up in your 30s, 40s, and 50s.

That's the conclusion of a recent study in the American Journal of Preventive Medicine.

When researchers from the University of California, San Francisco, followed approximately 5,000 adults ages 18 to 30 for 30 years, they found that doing the minimum recommended amount of activity — 150 minutes a week — is not enough to dodge midlife high blood pressure, dementia, and other chronic conditions.

It takes at least 300 minutes — an hour a day, five days a week — to stay healthy. And we say more than an hour a day is even better.

That's why we advocate (for all ages): walking 10,000 steps a day (that takes 90-plus minutes), doing 20-30 minutes of strength-building two to three times weekly, and doing sweaty aerobics for at least 30 minutes most days (you can do that with interval walking).

And don't sit down for more than an hour at a time. Get up, do jumps, walk up and down stairs; get your blood flowing for at least five minutes.

Then, as you reach 40, 50, 60, and beyond, you'll be able to maintain a rigorous schedule that lets you work and play in top form.