my shortest was up and down my block the day i bought my bike.
i rarely ride over 1 1/2 hours.
i want a road bike for endurance and the fact that i like to ride long distances.
going the other way, 35 miles is the longest i've ridden my mountain bike on a mix of trails and road.
at the same time, when i know i am only going out for 30 minutes or so, i push much harder. i use harder gears and really make myself work.