I think most people would agree that exercise is a good thing. There may be disagreements over how much, how often, what kinds and so on, but as with eating lots of vegetables, getting enough sleep, and managing stress, getting regular exercise is a pretty uncontroversial recommendation to support health and wellbeing.
Just like there is a point where healthy eating becomes unhealthy, there is a point where exercise becomes unhealthy. Over-exercise, compulsive/obsessive exercise, and exercise bulimia occur when exercise stops being a choice and becomes an obligation. If exercise is interfering with your life, occurring at inappropriate times/settings, or you continue to exercise despite injury or illness requiring rest, then engaging in exercise is no longer healthy for you.
As with disordered eating and eating disorders, the lines can be blurry. I can imagine a runner who is highly motivated to qualify for a marathon pushing through an injury because they're determined to make time. I wouldn't say that's advisable, but it's different from someone who is running to the point of injury because they're trying to burn calories or as punishment for something "bad" they ate. For one person, Crossfit might be positive, a fun way to push their body and see what it can do. For another, it might be dangerous. At the end of the day, it all comes down to your motivation for exercise.
Here's some questions to ask yourself to see if exercise might be a problem for you:
Do you not count an exercise unless it is a certain number of miles or length of time?
Is your primary motivator for exercise burning calories or burning off "bad" food you ate?
Do you exercise even when you're sick or have a fever?
If you can't exercise, are you afraid of gaining weight?
Do you workout in inclement weather?
Do you miss planned events with family/friends to exercise?
Do you feel guilty for missing a day of exercise or taking a rest day?
Do you work out through an injury you've been advised to rest from?
One thing to keep in mind is that exercise is a stressor. That's right. For all the stress relieving benefits you hear about with exercise (which it can be), exercise also increases cortisol levels temporarily. It's a bit complicated (if you really feel like digging into it, this post does a good job breaking it down), but essentially these short term bursts of cortisol are beneficial, while chronic high cortisol levels (from psychological stress), are harmful. And if your body is under chronic stress from work, family, relationships, under-eating, lack of sleep, etc, then you've already got so much circulating cortisol that you don't get that short term cortisol release that's actually beneficial.
Undereating paired with overexercising is particularly harmful. Here's some physical signs you might experience:
It's physically harder to do exercises that used to be easier.
You're moodier - more depression, anxious, angry and irritable.
You feel sore for multiple days after workouts.
Fatigue. No matter how much you sleep, you're always feeling tired.
Difficulty sleeping at night, or sleeping excessively and still feeling tired.
You're constantly catching colds and getting sick. Excessive exercise can weaken the immune system.
Low sex drive
Poor appetite, which may be paired with increase in body fat. This is a sign of your metabolism slowing to preserve energy and try to force you to slow down.
Loss of period or irregular periods
Dry hair and skin
If you notice a few or more of these signs, I'd encourage you to reach out to a dietitian or therapist experienced in disordered eating, or call the National Eating Disorders Association hotline.
That said, you don't have to experience physical symptoms of overexercise for exercise to be a problem in your life. Ask yourself if you'd be exercising in the same way, or with the same frequency or intensity if you felt comfortable with your body. If the answer is no, I don't know that what you're doing is best for your health and wellbeing in the long run.
Healthy exercise, or movement as I like to call it, looks different for everyone - kinda like healthy eating! For example, someone with a disability might not have the same kinds of activities accessible to them, but that doesn't mean they can't enjoy healthy movement in other ways that are. Independent of access to exercise, individual tolerance and enjoyment of physical activity vary. I have a few friends who are just constantly moving and finding joy in that movement. I think of my friend Anne for example, whose just a really active person, running marathons, kayaking, hiking on the weekends, doing power yoga, etc. It's fun for her, but her level of activity would burn me out! And she's able to take a break from it without stressing, like she did after her recent c-section.
Healthy exercise can be structured or unstructured, or a combination of both. Healthy exercise involves getting your heart rate up on occasion, but also doing slower, more restorative movement, like yoga or walking. Healthy exercise involves doing somethings that build strength, but that doesn't necessarily mean going to a gym to lift weights. How often or long you move, and what types of movement you do, can vary based on seasons of life, and that's OK.
Most of all, healthy movement is enjoyable. It increases your quality of life rather than detracting from it. It's something you do for pleasure, not punishment. Healthy movement is done out of love and respect for your body, not hate.
More on healthy movement: