Because exercise is nearly impossible with small children.
After work today my husband and I decided to go for a run on the bikepath. It started out great: both kids in the double jogger, the husband pushing as I fell a few steps behind (I am slower than him and need to pace myself).
Then all hell broke loose. Baby girl had the sun in her eyes and started screaming bloody murder and wouldn't stop. My son decided he had to get out and literally threw himself out of the stroller. I ran him over.
This resulted in me yelling at my husband to just go run. I pushed the kids and before I knew it I had baby girl in the ergo front carrier and my son was running in front of the stroller, getting in my way every 3 seconds and stopping to throw rocks in the water every 3 feet.
Needless to say, running didn't happen.
The thing is, it was still sort of okay. In the end we walked about 3 miles, stopped at the beach to throw rocks and look at the waves. The kids got fresh air. My husband got a run in. My son got a ton of exercise.
I have learned that even though it is totally inconvenient, and never goes as planned, it is important to just try. If you don't get out there and lug the kids along then you wont get any exercise at all, and some is better than none.