"A little more than you're doing already."
That's how much activity would do you some good.
That's where data comes in. You can't know what a little more is unless you know what you're doing now.
You could walk around with a notebook detailing every activity every day, or you could take the easy way out and get a tracker.
As electronics become cheaper, wearable technology can track your activity, and how that changes over time.
In fact, simply measuring something can help you improve it. There's an innate tendency to want to beat the high score whether it's Space Invaders, Candy Crush Saga, or daily steps.
As for Ten Thousand Steps, it was originally part of a 1965 Japanese marketing campaign to sell pedometers.
As it turns out, that number happens to work. It's not too high or too low. Distance-wise, it's about 8km, (or about 90 minutes of walking a day), and twice as much walking as an average office worker does in a day.
Being able to use a tracker like Fitbit or Nike+ to track your activity provides motivation to stay active.
Activity leads to healthy people. Walkable communities lead naturally to active populations.
If you live in a community where walking is impractical you will have be intentional about your activity. Tracking can help you know whether you're getting enough to meet your goals.
Don't knock yourself out over the number. Slow, consistent improvement will pay off in the long term. Keep beating that high score.