The Pediatrician Answers:
The type of exercise does not seem to matter with taking a walk at top speed was as good as running or biking or swimming. The goal is to get the base heart rate to double during the exercise. Anaerobic exercise such as weightlifting does not harm, but does not help physical fitness.
Why do we need exercise? We know it prolongs life, decrease number of infections, makes one feel happier and, probably most importantly, builds bones so that older people do not have as many fractures.