It's simpler than you think

Interval training doesn't require special training or equipment. In one study, walkers who added higher-intensity intervals to their walking program improved their aerobic fitness, leg strength and blood pressure. They did it by simply alternating between three minutes of fast walking and three minutes of slow walking for 30 minutes or more four times a week.

Peers who walked twice as long but at a moderate, consistent pace made minimal gains in fitness or other measures — similar to a comparison group that didn't exercise at all.