Several things begin to happen to your cardiovascular system in your 30s.
First, because the heart is a muscle, it begins to show signs of age like other muscles in your body. This is manifest in a decreased stroke count -- or the amount of blood pumped by your heart in a single beat. Second, the body begins to lose its ability to extract oxygen from the blood. To top it all off, this is the decade where cholesterol can begin to collect on artery walls and blood pressure can start to increase [source: Men's Health].
But it's not all as dire as it sounds. The early 30s also signal the time when endurance athletes typically hit their aerobic peaks, so with a little effort you can protect your ticking heart from the ticking of the clock. Your best bet is to try interval training: short but intense bursts of exercise where your heart rate climbs to 80 to 95 percent of its max, followed by a period of recovery before revving up again [source: Brant].