One big issue for which there is as yet no clear answer, is the question of whether it's the intensity or the amount of exercise which is more important.

One study involving 300 obese adults aimed to test this in a 6-month trial comparing those who maintained their usual activity level, those who did a low amount (around 30 minutes) of low-intensity exercise, those who did a high amount (around an hour) of low-intensity exercise, and those who did around 40 minutes of high-intensity exercise. All had five exercise sessions every week.

After 24 weeks of this, all the exercise groups showed smaller mean waist circumference and greater mean weight loss, compared to the control group, but there was no difference between the exercise groups in these measures. However, only the high-high group had lower 2-hour glucose levels than the controls, and only the two groups who did a higher amount of exercise had better insulin sensitivity.

Ross R et al. Effects of exercise amount and intensity on abdominal obesity and glucose tolerance in obese adults: A randomized trial. Ann Intern Med 2015 Mar 3; 162:325. (http://dx.doi.org/10.7326/M14-1189)