It's really hard if you work as well, or when your kids are toddlers and want to walk at a snail's pace. When my first was in a pram and I wasn't working I just walked every day - fast. I reckon I ended up quite fit. But as soon as DS2 was born it got much harder. I did have a double buggy for a short time but DS2 was a crier and it just wasn't the same, having to stop and try to comfort him or pick him up.
Then DS1 wanted to walk and it wasn't fair to tell him "no, you must stay in the buggy so I can walk faster to get some exercise". So that avenue to exercise stopped.
DH works stupidly long hours and leaves the house at 5.30am and by the time he got home at night he would put DS1 to bed while I did the crying baby and then we would have to sort dinner, cleaning up etc. I just couldn't face doing any exercise after 8.30, I was exhausted.
I suppose I could have done something at the weekend but I was either having my one lie in of the week or we went out somewhere as a family, or did food shopping in peace while the other had the kids etc.
DH managed to do some runs during the week sometimes at about 9pm but I had no desire to do that - too knackered and didn't want to run in the dark (plus bladder issues so not really able to run). Then when it should have got a bit easier to do it round all that, I went back to work, and so there was more housework etc to do in the evenings when the kids were in bed (or DH and I wanted to use the time to watch a box set together instead of exercising!_
It sounds like I'm making excuses but it was really really hard, I was really exhausted when I DID have a bit of time and just wanted to read a magazine with a cuppa instead. You have to LOVE exercise to want to be able to fit it in in those circumstances and I don't so......
Now I'm nearly 50 and I have more time but far, FAR less energy at the end of a FT working day and just fall asleep on the sofa.....