Exercise. Good for you. Too little, not good. Too much exercise, also not good. There is an optimal amount and I overdid it a few weeks ago and I am now paying the price. I thought my 55 year body could take the stress of increasing my running mileage which had been 15-20 miles per week, to 20-25 a week. It doesn't sound like much but to go from 17 to 23 is a 35% increase. To add injury to injury, I went from running flat courses to running a 6 mile loop that included a 1.9 mile hill, the last 400 yards at a serious incline. That particular course I ran 3 times in the last 8 days before my injury. 
So you may be thinking muscle pull. I wish. Runner's knee? Been there, done that. No, my glamourous injury is an inguinal hernia. I had been feeling pain for a while in the groin area but it wasn't until playing ice hockey after those arduous 8 days of running did it magically appear in the shower. This is my 2nd one. It will require surgery to fix and that is on the horizon in a few weeks. Moral of the story, moderation in all things. After all apples are good for you but would you eat 9 a day? Sleeping is good. Sleeping for 12 hours - not good unless you are an infant. Moderation in all things healthy. The suggested dose of exercise is still (basically) 30 minutes a day, with your heart rate approximately 70% of maximum, and 3 - 4 times a week.
One more example. Think of the benefit you get from running 3 miles. That benefit is a lot more than the same 3 miles that occur between miles 17-20 of a really long run. It's the same distance but one will give you vitality and one will give you bragging rights with a moderate dose of risk and physical breakdown.
Be sensible and be safe, and that includes a gradual increase in intensity. A lesson I knew but chose to ignore.
