For me, activity had to be incorporated.
I had a terrible time keeping up an exercise routine with small children. I did best by just being busy, and not in the kitchen! Walk a lot. Stand rather than sit. Exercise by putting on fun music and dancing with the kids. Break a sweat at least once a day!
Time to myself was very difficult, but time with the kids was always available! My biggest problem during the small kid years was eating triple: while I cooked, on my own plate, and finishing the kids plates when we were finished. Eeek!
Did you ever have time to take the kids somewhere and get an hour workout in? Or go for a walk solo.. go to a quick class? Regardless.. doing what you can counts!