Like most people have said, Google assumes a flat 10 mph, so you have to adjust for your own speed and route. Their routes are pretty questionable, especially over longer distances. I've had them route me 15 miles out of my way simply because their was an MUP there. If you know your route, Map My Ride will be more accurate. They say Google Maps bike feature is in beta stage, I say Llama stage: that's what you'll need, a Llama, sherpa, a couple wool blankets, some extra food....

Marc