Interval Training

Several physiology labs study interval training, a form of exercise which varies intensity of an activity during a cardio workout. Any activity like running, swimming, cycling etc. can be used but exercise bikes are often the most practical for many people.
Regular cardio workouts increase cardiopulmonary reserve, help reach or maintain optimal weight, support good sleep and increase longevity. There is good evidence that high levels and more time spent provide better results but that even modest brisk walks several times a week are very helpful.
A Danish research lab documented excellent results from this regime:
   short warmup
   30 seconds of normal exertion
   20 seconds of greater exertion 
   10 seconds of maximal exertion
   short cooling off

  https://www.ncbi.nlm.nih.gov/pubmed/25439558

This is repeated up to a total of 10-15 times in sets of five separated by two minutes of very light activity. Three times a week is more effective than daily workouts just as is true for building strength through weight training. As with all workout programs numbers and intensity of the workout should be gradually increased until 10 to 15 reps can be done comfortably. 
Their comparison with other cardio training methods showed superior conditioning results with relatively little time invested.
I use this method and find it easy to follow- not too painful or or time consuming.
Mayo Clinic researchers tried different exercise regimes (vigorous weight training; light weight lifting plus moderate intensity exercise bike; interval training) on young and older subjects who allowed muscle biopsies to monitor results. All activities improved fitness and ability to regulate blood sugar. Cellular genetic mitochondrial changes reflecting cellular health and younger apparent age were much greater in those who did interval training. Remarkably, these effects were much more in older people. Interval training can now be called "a fountain of youth." 

http://newsnetwork.mayoclinic.org/discussion/mayo-clinic-discovers-high-intensity-aerobic-training-can-reverse-aging-processes-in-adults/