I am not an avid exerciser but do get out on walks most days with my little ones. We often walk to nearby parks, they play, and then we walk home. I'm almost always pushing one, if not both, in a stroller on those walks because of the distance.
I read this thread yesterday and didn't think I had anything to contribute until I was out enjoying a wonderful bike ride today.
I use a front-mounted seat for my young toddler (we started both kids in it after they were a year old). Now that I have a pre-schooler who can bike independently but not for long distances, I use a tag-along (trail-a-bike) on the back of my bike. Both kids love our rides. I don't go out of our neighborhood because I'm not interested in being near much traffic, so my rides aren't super-long, but they are certainly giving me exercise as I pedal 70 pounds of kids and 60 pounds of bikes and myself up some good hills.
I was an avid biker in my youth so felt very comfortable on a bike before adding the contraptions. The front-mounted seat caused me no problems with stability (I just had to lower my own seat a bit to make sure I could easily put my feet on the ground when I needed to). The tandem attachment with a big girl on it makes me more wobbly (especially when she wobbles on purpose), but we've been fine so far. It's been a great form of exercise and entertainment, and it's also nice on the occasions when my husband takes the kids out on a pretty Saturday afternoon and leaves me home by myself.