UNDERSTAND ONE THING RIGHT NOW: Your body needs your attention. Despite what the infomercials say, not even hair is replaceable, so no matter your age, your long-term fitness plan had better start today. There are three keys to keeping your system working—and winning—for decades: (1) Protect your joints, (2) maintain muscle mass, and (3) accomplish numbers one and two with a realistic training program—like the one I've devised here—that jibes with your busy life.

Why focus on joints? Where you bend is where you break. Your hinges are your body's Achilles' heel: When allowed to weaken, they become unstable and susceptible to an injury that will leave you unable to train hard and stay active. That in turn causes you to lose the lean muscle mass that keeps your metabolism high and the midlife pudge in check. And becoming overweight and out of shape leads to bad habits that cause your fitness to slide even after an injury heals. Maintain joint health and you'll prevent this vicious cycle from ever starting. To keep your hinges as strong as a vault's, you need a workout that simultaneously engages multiple parts of the anatomy. Biceps curls and calf raises are out, overhead presses and squats are in. Working several muscle groups at once means strengthening your connective tissue in the ways you actually move, which stabilizes and protects joints. At the same time, you fight off the natural lean-tissue loss that starts at around age 35. Core strength protects the complicated mechanics of your lumbar and pelvis, preventing lower-back pain, which is why crunches and planks are also included.

THE FOLLOWING WORKOUT is intentionally low-tech, so you can do it at even a minimally equipped home gym. Two to three times a week, cycle through the full set of exercises, resting for 45 seconds between each, and then repeat the sequence. Go light for a week or two until your body adapts to the range of motion, then increase weight, using enough resistance to make the last rep of each exercise strenuous (but not impossible). As you get stronger, add resistance or time to the exercises so you keep progressing.

1. SQUAT
What It Works: entire lower body, lower back, core Form: Stand with feet slightly wider than shoulder width apart and extend your arms for balance. Lower your hips, as if you're about to sit. Stop when your thighs are parallel to the ground, then rise back to standing. Keep your torso as vertical as possible throughout so you feel your weight centered over your heels, not over the balls of your feet. 10 reps Advanced: Add resistance with dumbbells, a barbell across your shoulders, or resistance cords.

2. SHOULDER PRESS
What It Works: shoulders, upper chest, triceps, back muscles, core Form: Stand with your feet shoulder width apart, with dumbbells in your hands. Raise hands to your shoulders, palms forward. Press hands straight up, pause, then lower to shoulders. 10 reps Tip: A standing shoulder press is better than a seated one, because you have to engage muscles from head to toe.

3. PULL-UPS
What It Works: lats, back, shoulders, biceps, forearms Form: Grasp the bar, palms facing forward, with your hands slightly more than shoulder width apart. Pull your body up until your chin clears the top. Lower yourself until your arms are straight, but don't lock your elbows. Aim for 10 reps, or as many as you can complete in one minute. Gym Option: You can substitute a lat pull-down for this exercise.

4. UPRIGHT ROW
What It Works: shoulders, back muscles that stabilize the spine, core Form: Use dumbbells, a resistance cord, or a barbell (hands close to the center of the bar). In all cases, your palms face inward. Stand with your feet hip width apart, knees slightly bent. Keeping your back straight, chest high, and head up, pull up with both hands simultaneously. Keep hands close to your body and elbows high as you bring your hands to mid-chest. Lower your hands back down. 10 reps

5. REVERSE LUNGE
What It Works: hamstrings, quadriceps, buttocks, hip stabilizers, lower back Form: Standing with feet together, step back with your left foot and drop your hips until your right knee is bent at a 90-degree angle and positioned above your right ankle. To reverse, push off with your left foot and drive with your right leg. 10 reps, then switch legs and repeat Advanced: Hold dumbbells in your hands or a barbell across your shoulders.

6. TWISTING CRUNCH
What It Works: Obliques and transverse abdominal muscles Form: Start by lying on your back with your knees raised and your hands behind your neck. Crunch your left elbow toward your right knee, bringing them together over the center of your body. Return to the starting position and repeat with your right elbow toward your left knee. Alternate nonstop for one minute.

7. PLANK (SIDE AND PRONE)
What It Works: Your entire core and lower back Form: Lie on your left side, with your legs, hips, and shoulders in a straight line. Prop yourself up on your left forearm so your elbow is directly under your shoulder. Lift your hips off the floor so you create a straight line running from your right shoulder to your right ankle. Alternate sides. Prone version: Start by lying on your stomach. Support your upper body with your elbows directly under your shoulders and your forearms flat on the ground. Hold your body in a straight line from your shoulders to your heels. Hold each position for 30 seconds to a minute. Advanced: Use your hand(s) for support. This also works your chest, shoulders, and triceps.

Carmichael Training Systems founder Chris Carmichael is Lance Armstrong's former coach.