We typically try to get at least an hour of physical activity in each day for the kids. Even if the physical activity consists of playing on the swings! Walking to and from school is a great way to get it in. Our school really promotes walking and biking. They have several Walk to School days a month where they encourage the kids to walk together. Even people who typically drive their kids drive them to a drop off spot and then all the kids walk from there.
I am a total abnormality in my social circle though as we spend quite a bit of time at the park playing or walking or biking even when its, gasp, raining or cold! I want being outside in nature and getting wet and playing hard in the mud to be a huge part of childhood for my kids. Every time I post pics of us outside playing in the rain or mud or whatever, I get a ton of comments about how cool I am to "let" my kids do that.
I feel you on the bike thing. I learned how to ride a bike last summer and it was so freeing for me. I have horrible balance complicated by MS now but once I got over the fear, it was awesome. I'd always just accepted that I couldn't ride a bike until one day my dd asked why didn't I just try. Total duh moment for me! I also learned how to swim last summer because of the same thinking. I was tired of defining myself to my children as a person who couldn't do this or that.