Seeing the example of their parents making healthy choices and keeping active is the foremost influence on both children and teens. I have seen mine become immersed themselves in running and strength training and biking as their dad and I increase our own efforts. I've always been careful to avoid negative body-image talk around my girls, because the media and society is so full of them already. I want my kids to be healthy in body and mind, not sickly-skinny or artificially pumped up.
The only very deliberate decision I made concerning my kids' outdoor time was probably the result of having the lowest opinion of television, even for adults (think of how often Really Important People, like Sting or Mother Theresa, bothered with tv watching). Don't you want your life to be full of Really Important Activities? You only get to do this once, you know. *
So that leaves one with a lot of time to just enjoy life...and to muck around outside, especially in nature. My first two children think they were the luckiest ducks in the family; they got to live on the coast for a few years of their lives. Yes, the ocean was swell, and I miss it, a lot. The woods and the creeks and the rivers of Iowa are also wonderful. For one thing, the bike path along all of the above is litter, dog doo and algae-free. All of those were the obstacles keeping me on my toes and oft my double stroller on its side wheels as we navigated those early years of exercise with mommy. A run along the beach, in the dull, gray winter, but nevertheless, with the spray of the ocean and the smell of the salt water...and my feet getting tangled in mounds and mounds of sea-weed as I try to simply jog along. I love our creek down the street and seeing the woods and prairie change with each season; it keeps us coming back all the time (will we see Mother Fox today? have the tadpoles hatched, is the woodpecker back?), which keeps us moving.
Find a reason to go out and move. Maybe you start to say hello to the same man walking his dog each day and keep a treat in your pocket for the puppy. Are there construction projects happening in the neighborhood you could check on, a favorite tree to visit where their father saw a fairy years ago? Negotiate less written math if they will march out the times tables with you or find words that begin with every letter of the alphabet along the way.
It's rainy, snowy, too hot to trot? Dance inside to good music, hold a weekly yoga session, let the kids who are in kung-fu or swimming teach you how to do push-ups and leg lifts the way they do them in class. My children love to teach and train me...and I need it! Try workout online videos or DVDs from the library or from the thrift store, they get tossed a lot. Watch and try the video together, help each other improve your posture and accuracy.
Make movement and healthy eating part of your life and enjoy it. The kids will follow your example and make their own way to lifelong fitness.
*I am not a big joiner, but I joined two organizations; CCFC (Campaign for a Commercial Free Childhood) http://www.cbf.org/ncli/action/about and No Child Left Inside: http://www.cbf.org/ncli/action/about. Check them out if you are interested.