What is the single best thing we can do for our health?

I came across this video a while ago called 23 and 1/2 hours: What is the single best thing we can do for our health? It's a bit long (nine minutes) but it stuck with me. If you don't have time to watch it, the basic message is that thirty minutes of exercise a day will make a huge difference in your health. It's hard to find the time, particularly in the winter with days getting dark by 5:00. I'm so much less motivated to run after it starts getting dark. But this video was a good little push to help me realize it's worth it.