I've gotten a lot of mileage out of this old Nike ad that ran in Runner's World a long time ago. For some reason, it just really appeals to me.

Part of it got cut off but the full text goes like this:

Ten million decibels loud, And it doesn't care you're tired. Or it's your birthday. Or some holiday honoring a saint. So, though you'd rather not, you start down the road again. The road, when it calls, it screams,

Just Do it!