Why do you want a road bike? Do you plan to ride with other roadies, or just by yourself? Do you want to race? If you just plan to ride alone for exercise, there's no immiment need to change bikes. If the mountain bike is comfortable to ride, just keep using it. If you want to ride mostly on pavement and want to go farther or faster, aside from the obvious answer of doing it through practise and exercise, you can also replace the knobby off-road tires with slicker street tires.