I have tried ziplining a few times while vacationing. It is always fun, but always too short and certainly never scary. Time to vacation in Nepal I guess.
The zipline pictured above, the ZipFlyer, has a vertical drop of about 2,000 feet (600 meters) and a total distance of about 6,000 feet (1.8KM, 1.13 miles). So you go over a full mile on a zipline and reach speeds of 87MPH, that would be both long enough, and scary enough for me.
Video of the ZipFlyer zipline in Nepal after the break.