hi ... even 10 minutes a day would be a great start; the problem with trying to do more than this if you aren't used to it is you can end up with injuries which then prevent you from being able to do any exercise. Also ... if you haven't done it already; getting a checkup with a doctor before starting a new exercise program is a really good idea

with exercise it is great having a starting point then as you can do the 15 (or 10) more easily build on that ... either by going a bit longer or adding more intensity; go gradually though

another thing that can help is breaking the exercise up e.g. do 10 mins in the morning; then another 10 later in the day

great job on doing this