The key success factors are INTENSITY and Consistency
If you’re just starting out, do 30 minutes, 3 days per week of anything, walking, elliptical, treadmill, recumbent bike, bicycle, light weights, machines, cable machines, anything. Moderate activity. That’s enough, even for formerly experienced exercisers. Try to have a day or two between sessions. I know it may not be difficult. There’s plenty of time. You’re here for the long term. You’re not 20 any more. Surprise!!! Be smart. You’ve earned the right to do it right the first time and NOT have injuries.
If you have been at it for a while, no more than 45 minutes, 3 days per week. Mix weights and cardio, but keep it less than an hour.
If you’re experienced you may want to exercise 45 mins. – 1 hour (but no more, ever).