I believe physical activity is important if you want to have a decent length of life and be around to see your kids and grandkids. Healthy habits need to start young.
I require then kids to each pick a sport where they train 2-3 hours a week out of the home. I nag them enough about academics that I don't want to force exercise.
On top of that, they may practice on their own, we do walks and bike rides, etc.
We also do Boy Scouts. I was scoutmaster for years and brought DD along. I just got permission from the new troop to bring her sometimes. Yesterday, DS2 and I did an 8 mile hike through a wooded canyon with a beautiful stream running through it. (Yeah, most of the boys ended up in it at some point...lol)
In the late summer and early fall they play soccer, so I let that be it. Seeing as how they are constantly running and playing I'm okay with no other formal stuff. In the winter we go for runs and hikes if it is decently warm and play wii fit if it is a blizzard or what have you. In the spring and summer we do all kinds of "gym" activities: swimming, running, basketabll, tennis, volleyball, and of course hiking and biking. We still follow my mom's rule that if the sun is shining in the summer we leave the house after breakfast and stay out until dark.
I required my kids to do something at least thirty minutes total a day. It didn't matter what it was. When they got to high school level I had them take classes. They found their true passions for a particular sport that way.
I think it is just as important as academics. He does baseball about 6 months out of the year. If it is nasty outside we play on the xbox Kinnect, if it is nice we go for walks, go to the park or just run around outside.
I have to make it as important as reading and Math. My sons take a PE class once a week, karate twice per week, hiking, and of course chasing our chickens. My youngest and I get up in morning and go for run/walks together.