They say that showing up (or, in this case, putting it in writing) is half the battle—so for that, congrats on a big first step. Now, for your question: Science says that at 40 you are definitely not too old to benefit from a regular exercise routine. Great news, right? Drop and give me 20!
But back to the research: A recent study found that senior men, ages 55 to 70, who had started a "relatively intensive" endurance training program after age 40—either cycling or running more than seven hours a week—performed similarly on cardiovascular tests to those who had been regularly working out since before age 30. And both groups, not surprisingly, performed better than a third group of seniors who had never exercised for more than two hours a week in their lives.
Specifically, the post-40 and the pre-30 exercise adopters both had lower resting heart rates (58.1 and 56.8 beats per minute, respectively) and higher maximal oxygen uptake (44.6 and 47.3 ml/min/kg) than the still-sedentary seniors (69.7 beats per minute and 33.0 ml/min/kg). Scans also showed that the exercisers had bigger atria and left ventricles, thinner vessel walls, and better diastolic function than the non-exercisers.
"Despite biological changes with age, the heart still seems—even at the age of 40—amenable to modification by endurance training," says study author David Matelot of the University of Rennes in France. "Starting at the age of 40 does not seem to impair the cardiac benefits."
Of course, just because something can be beneficial to your heart doesn't mean it's automatically safe for everyone. Before you dust off your old Schwinn and log your first seven-hour-workout week, be sure you have a clean bill of health from your doctor—and consider talking to a personal trainer or certified fitness instructor about how to get started. If you're coming from a fairly inactive lifestyle, it's best to ease into a much more athletic one. "There's no need for a high level of training for many hours a week," says Matelot. "Using the stairs rather than the elevator, or gardening regularly, can also be beneficial."
Oh, and for those of you under 40? This is no excuse to sit back on your laurels until you're officially Over the Hill. Matelot also says it's still best to start (and keep up with) a regular exercise routine as early as possible—not just to keep your heart healthy, but also to improve bone density and muscle mass and reduce cancer-causing oxidative stress as you age.
Bottom line: Get your doctor's okay, then get on the treadmill. Starting a regular running or cycling routine—even after age 40—can improve your heart health for years to come.