There are many positive effects of physical exercise. Some of the most commonly cited are improvements in cardio-vascular health, the possibility of reducing cholesterol and increasing vascular efficiency, lowering blood pressure, etc. It can also lead to weight loss, higher muscle density, all kinds of thing that are important in a well-functioning body.
There are also mental benefits as exercise has been shown to improve cognition and improve performance in all kinds of mental tasks for people of all ages. It has also been linked to decreased rates of dementia in older people as well as improving and sometimes reversing the effects of dementia or Alzheimers in older patients.
The appropriate level of exercise is of course unique to each person and depends on their level of experience, the health of their heart or if there are other pre-existing conditions. Simple principles of moderation and self-evaluation can go a long way towards preventing negative effects of too much exercise.