Yeah I understand the obsessive compulsion to exercise. For me it’s not been about being slim (although I did many years ago have an active eating disorder) but I had a point in my life where I absolutely got obsessed with my stats. It was not healthy - although I was physically very fit.
I still regularly have to have words with myself , because I know the benefits of exercising aren’t about how fast you are or how hard you push yourself. And yes you’re right it is possible to over exercise, that’s when you get tired, pick up injuries and infections/illness, that’s the opposite of health.
I also had half marathon urges - I’m hoping I can resist. I did a few that were good but last years was horrible and I need to remember that.
I guess the only thing I can say is keep on having little pep talks with yourself, remember what you want to achieve. Be careful (if this is a problem) who you exercise with or around too, avoid the competitive types I’d say from experience.