You will gain the most benefits from your exercise program if you follow the "stress and recover" training principles that competitive athletes use. A study using accelerometers to measure the physical activity of more than 90,000 healthy people over six years found that the more and harder they exercised, the less likely they were to suffer heart disease (PLoS Medicine, December 2, 2021). There was no maximum amount of exercise that increased risk for heart attacks or heart disease. The American Heart Association (AHA) recommends that adults should participate in either 150 minutes per week of moderate-intensity aerobic exercise, 75 minutes per week of vigorous aerobic exercise, or some combination of the two.

Researchers agree that exercise helps to strengthen your heart, prevent diseases and prolong life, but exercisers need to know the signs and symptoms of exercising too much, and know when to slow down or take a day off to avoid injuries.

Stress and Recover
If you want to improve your strength and endurance, you should train by stressing and recovering. Don't work out at the same intensity every day. You will get more benefit if you exercise harder on one day, feel a little sore on the next day and then go at a slow pace for as many days as it takes for your muscles to feel fresh again.

On your stress day, warm up by starting out at a very slow pace, and then alternate a series of going a little harder and faster until you feel a slight burning or tightness in your muscles. Immediately slow down and go at a slow pace until you feel fresh again. In the beginning, you may want to pick up the pace for only 5-10 seconds ("intervals"). As you improve, you can work up to 30-second "intervals" at your harder pace. For non-competitive exercisers, you don't ever need to stay in an "interval" for more than 30 seconds.
• You recover by going at a slow pace for as long as it takes for your muscles to feel fresh again. There is no advantage to time-limiting your recoveries. When your muscles feel fresh, you can pick up the pace and slow down immediately when you feel a burning or tightness again in your muscles. Novice cyclists can do 5-10 pedal strokes, and then go slow for as long as they need for recovery. Novice runners can start out by taking only 5-10 running steps. Swimmers can do short bursts of their favorite stroke, and so forth for any activity you choose.
• Continue to alternate these slight pickups of intensity with slow recoveries until your muscles stop recovering as soon as you slow down. Eventually you may want to work up to 30-second intervals followed by untimed recoveries. At your peak, your stress workout can include a 10 minute warmup, a maximum of 30 minutes of controlled intervals, and a 5-10 minute slow recovery. A typical hard-interval workout would include 20 to 30 intervals of 10 to 30 seconds each, with untimed full recovery between each interval.

Your recovery days ("easy days") should always be governed by how you feel. On the day after a stress day, your muscles will probably feel slightly sore and you are supposed to recover by going at a very slow pace. The muscle discomfort is called Delayed Onset Muscle Soreness (DOMS). You warm up by going very slowly, and if your muscles don't feel more comfortable after a 5-10 minute warmup, take the day off altogether.
• If your muscles feel better after your warmup, go at an easy pace for as long as you like and stop your workout when your muscles feel any protracted discomfort. You are never supposed to work through pain. Always stop if you feel fatigue or muscle tightness or discomfort.
• Continue to take slow recovery workout days for as many days as it takes for your muscles to feel fresh after you warm up for 5-10 minutes. Only then should you take your next hard stress-day workout. Most exercisers follow each stress day with one to three or more recovery days before they take their next intense workout.

How Can You Tell if You are Exercising Too Much?
There is no laboratory test that will tell you that you are overtraining. You should take days off or markedly reduce your workouts if you suffer:
• any injury
• muscle soreness that does not go away after you warm up
• any illness
• continued difficulty falling asleep or staying asleep
• continued loss of appetite
• continued fatigue or loss of energy
• recurrent or prolonged infections
• a persistent increase in resting heart rate of more than 10 beats per minute
• decreased muscle strength, endurance, or ability to perform and recover from your regular workouts

Heart Problems in Elite Athletes
If you follow the rules for recovery and back off if you have warning signs of overtraining, you should gain the health benefits of an exercise program. Countless studies have shown that exercise helps to prevent heart attacks, but some researchers have found scarring in heart muscle and increased plaques in the heart arteries of people who have run many marathons or triathlons, resulting in news headlines warning that "too much exercise can harm your heart." A 10-year review of 10.9 million regular distance runners found 59 instances of sudden death from heart stoppage, for an occurrence rate of about one in 200,000 participants (NEJM, 2012;366:130-40).

At this time, the prevailing opinion is that:
• heart attacks are the result of sudden complete obstruction of blood flow to the heart caused by clots formed by plaques breaking off from arteries leading to the heart,
• plaques form in arteries primarily from a faulty diet, and
• exercise helps to prevent heart attacks by stabilizing plaques so that they do not break off to start the process that leads to heart attacks.

My Recommendations
I believe that every healthy person can benefit from a regular exercise program, provided that they do not do so much exercising that they feel fatigued or sore much of the time. To increase strength and endurance, you should follow a stress-and-recover exercise program, and know the signs and symptoms of overtraining. In addition to trying to exercise every day to strengthen your heart and stabilize the plaques in your arteries, you should follow a lifestyle that helps to prevent plaques from forming in your arteries in the first place:
• eat lots of vegetables, fruits, nuts, beans, whole (un-ground) grains and other seeds
• restrict all sugared drinks including fruit juices, sugar-added foods, red meat, processed meats and fried foods
• avoid smoke, alcohol and recreational drugs
• avoid being overweight
• keep blood levels of hydroxy vitamin D above 30 ng/mL

Caution: Before you begin a new exercise program, if there are any questions about your health you should check with your doctor, particularly if you have chest pain or any heart attack risk factors or heart problems.