A measure of your aerobic fitness is your VO2 max. Improvement in VO2 max is directly related to the frequency and duration of training. An indirect way of testing ones VO2 max is to measure heart rate. This is significant. It means that exercising for a longer duration at low intensity will produce the same improvement as exercising for less time at higher intensity, if the total number of calories expended is the same or similar.
Improvement is achieved if the exercise is intermittent or continuous.
So, exercising once for 30 minutes or 3 times for 10 minutes produces roughly the same benefit. One should exercise three days a week to maximize improvement. Students who exercise more than 5 days per week have no improved fitness for the added extra days of exercise. Cross training is best to target the various major muscle groups. High impact activities are not wise for beginners because of injury risk.