For the bike you currently have, I would just add bar ends. If you were going for a different bike that would be equally suitable for a flat/riser bar or drop bars, then the question becomes a little more involved (for me). At any rate, given the distance you mention, the flat bar with bar ends should be sufficient. Also, in case you're seriously looking at drop bars for this bike, reusing the same brakes and shifters is not as simple as you might think (or even possible). Adding bar ends would be fine, and you definitely wouldn't need to worry about the brakes and shifters. FWIW, I ride 18+ miles RT each day on a bike with a flat bar and risers or another bike with drops--mostly rolling terrain. No problems riding either bike. Riding either bike is more enjoyable than driving a car.