HIIT: high intensity interval training
In conclusion: when exercise time is limited do shorter but more intense exercise bouts. When you have more time (like on the weekends) do longer exercise bouts at a moderate, or easier intensity. .Both are important. If you don't have time for "a workout" then do 5 flights of stairs, 3-4 times a day; that counts as exercise
There are data to suggest that moving consistently thoughout the day brings similar reductions in risk as doing a 30-min brisk walk in the morning and then sitting the rest of the day (people who do this are called "active coach potatoes").
As it turns out, doing some high intensity exercise becomes more important as we age.
A 2017 study published in Cell Metabolism compared genetic changes in a group of younger (<30 years of age) to older exercisers (>64 years of age) using different types of exercise routines; a HIIT routine (4 min at 90% of max, repeated 4 times) and moderate exercise for 30 minutes a few times a week combined with some resistance training.
The HIIT training resulted in beneficial changes in 274 genes in the younger group and almost 400 genes in the older group. Thus, the decline in the cellular health of muscles associated with aging was "corrected" with the more intense exercise and responded more robustly in the cells of the older group compared to the younger group.
One study had sedentary women do vigorous stair climbing (3x20sec all out followed by 2 min of recovery) 3x per week for six weeks.
After the six weks, they found that the women had significantly improved their cardiorespiratory fitness.
This study was based on results of other studies which compared high intensity interval training (HIIT), where the total volume of the training is quite low but the intensity is high, to longer periods of exercise at lower intensities.
Consistently it has been shown that one can achieve substantial health (like increased insulin sensitiveity-a good thing) and fitness benefits (like increased aerobic capacity) from less time spent exercising, if the activity is quite intense. One study showed that 12 weeks of HIIT improved fitness and insulin sensitivity to the same extent as moderate exercise that took 5x more time.