When the kids are in bed, before I go to bed, I do my Ashtanga yoga. That is tough, but relaxing at the and, so I can easily rest after.
When the band was setting up before a concert I did push-ups. Waiting for the train I did push-ups. When I was waiting for class to start I did push-ups.
Anytime I was bored or stressed, I did push-ups.
Soon push-ups weren't enough, and I looked for more exercises …
And you should at least try to exercise a few times to understand that you love it, it makes you full of energy and ready for the day.
Just find your kind of exercise. Mine is running.