This is how I have fitted in exercise including when a single parent and having small DCs.
Running. You absolutely can have the DCs with you. They can either run with you (which I appreciate is not likely to lead to a nice rhythm and distance for you) or they can just be with you whilst you run (scooting or on bikes) Or you take them somewhere and run in big circles around them. For example they play on the swings and slides whilst you run laps round the play park. I’ve parked a baby under a tree in the park and run laps round and round many a time. You set up races for older kids. Relays. Sprints. Rolling down a hill races whilst you run up and down it 20 times and until you are beetroot in the face, Whatever.
Sports
Why can’t you play football/rounders/badminton/tennis with the kids? You know there is more running for the ball etc to retrieve it than actually playing, usually, so it gets your running up. You can take it seriously as exercise and they can play the game. When they get bored, run laps round them as above!
Long walks
Whilst I very much understand they aren’t as easy when you haven’t the DC, they are still possible. We pack at least two picnics and aim for a whole day walking, ending up somewhere for tea.
Classes with DC
There are classes you can take the DC to here, and there are still more that don’t involve them as such but where there are often some kids at the side with a screen of some type, perhaps that’s not ideal but it is done often and doable.
Dancing with DC
We have a disco light and have full on mad discos at home, I’m meaning 7k+ steps worth of dancing, and fun whilst you are at it.
I guess my best tip is to stop thinking about exercise and activities/caring for the DC as being separate things, and also suggest the mantra that every little helps - ten minutes of exercise three times a day is a good place to start, and often a much more doable target than getting out to a class.