As others have noted, how much exercise you do depends on your current level of fitness. If you find that working out 20 minutes three times a day is too much, then why not try exercising for 10 minutes three times a day ? No one says you have to exercise for 60 minutes a day to be healthy. Although, yes, many experts say that people should be active in some way for at least 60 minutes. but that is because many people have very sedentary lives.
Have you been sedentary and are you trying to become more fit ? If so, then you want to slowly ease into a regular exercise routine so that it will become just that, a routine. Before my 86 year old aunty fell and broke her hip a couple of years ago, she would routinely walk 2-3 miles a day.
Walking is a wonderful way to stay fit as well as active. Why not take a daily walk as part of your exercise routine ? You don't have to kill yourself to be fit or healthy.
And if you'd like to learn more about fitness and older people, you might want to check out this website. Tufts University did a landmark study on strength training for older adults. They found that age is not a barrier to increasing lean muscle. You don't have to lift heavy weights. They have a simple series of ST routines posted that you might want to try. growingstronger.nutrition.tufts.edu/index.