Depends on which brake, and the route.
I'm much more likely to cycle with no working rear brake, than no working front brake (I'm assuming this isn't on a fixie). If there are hilly bits, you need a rear brake to slow down, since the front will tend to snatch.
Assuming it's the rear brake that's gone, and it's not all steep downhills, I'd be inclined to risk it, but take things very carefully.