That's a nice question, and really logical. However, it really doesn't translate. There are too many variables when actually out on the road.
However, I can say this, that's a good start. You should learn to ride. If the stationary bike is actually accurate, you're riding 3 minute miles. You aren't going to win anything doing that, but it's a good start. If you could ride 10 miles in 30 minutes first ride out, with good equipment and training you could pick up the pace from 20 mph maybe up to 25mph.
However, I can say this, that's a good start. You should learn to ride. If the stationary bike is actually accurate, you're riding 3 minute miles. You aren't going to win anything doing that, but it's a good start. If you could ride 10 miles in 30 minutes first ride out, with good equipment and training you could pick up the pace from 20 mph maybe up to 25mph.