31% to 39% is not a huge gain for triple the time!  I would say the 150 minutes a week is the sweet spot.

"Those who met the guidelines precisely, completing 150 minutes per week of moderate exercise, enjoyed greater longevity benefits and 31 percent less risk of dying during the 14-year period compared with those who never exercised.

The sweet spot for exercise benefits, however, came among those who tripled the recommended level of exercise, working out moderately, mostly by walking, for 450 minutes per week, or a little more than an hour per day. Those people were 39 percent less likely to die prematurely than people who never exercised."

150 minutes a week is 22 minutes a day.

450 minutes a week would be wonderful but it's hard for most people to find an hour a day for exercise.