Peta Bee covers J Strength and Conditioning about minimal fitness sessions in the Times. 'If you exercise regularly you can get away with doing much less and still stay fit':
Cardio - 13 minutes x2 per week at 75% maximum heart rate. Or, 30mins around 50-60% maximum heart rate. In other words shorter and more intensive.
Weights - 6 exercises, performed slowly, once weekly, one set of repetitions. Oldies may need 2 sessions.
Balance - important skill for oldies. Balance on one leg every day increasing to 30 seconds. Eyes open to start, then closed. 'Then try squatting down on one leg or moving centre of mass by swaying on one leg with eyes closed'.
Step counts - you can get away with 4500-5500 p. day according to JAMA (based 17000 older women) and US National Institutes of Health.
Warm up - slow version of the exercise you are going to do, then maybe 5-10 seconds of effort before starting.
Cool down - don't stop suddenly, gradually reduce intensity.
So my usual advice - do something and do it regularly.