Some simple math would give you a good estimate and on a flat course (assuming no wind) you wouldn't have huge variations in speed to consider. I don't know what a "good" speed is on flat ground (most of my rides are always at least a little up and a little down) but in my less than ideal shape I can do 20 mph or better on flat-ish roads. Never tried to hold that for 12 miles because I always hit a hill (so don't start out at top speed would be the point). I'm guessing I could do it in around 36 minutes which would probably qualify as a "not so good time". 30 minutes would be an average of 24mph over the course which is probably a decent time.