Once you've pressed the Get Directions button, scroll down to see your directions. Sometimes you'll see suggestions for multiple routes with the time estimates for each. This might be a good time to compare your route to traffic conditions by pressing the Traffic button on the right (on top of the map view). This isn't available in all areas, but where it is, it should help you pick a route.
If you know you want to use an alternative route that isn't offered, you can just drag the path anywhere you want to reroute, and Google Maps will update the directions on the fly. This is especially handy if you know the road is under construction or the traffic is congested along the standard route.