what are the positive effects of exercise and appropriate levels to decrease the risks of negative effects?

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.