Thanks for the tips everyone. I used some of them today. I got us all ready in the morning and when DD2 was ready for her nap, strapped them both in the pram and walked to the local duck pond and park (we live in a very hilly area so that was good too). DD1 fed the ducks and then played on the playground. In between playing with her I ran up and down a hill a few times and did various aerobic moves whilst waiting for 'my' turn to go down the slide. I would have looked like a right idiot to anyone looking out their window! DD1 tried to copy me for a bit and then asked me to stop and have a rest lol.
I'll need to try and do it more often.