If you're going to do your metric century, you might as well work to find the route that you can get the most out of it. Crater Lake, Mt Scott to Steamboat.
70.24 miles (113 km, went over a bit).
Drop from 7,750 feet elevation down to 1,153 feet.
For a total elevation loss of 6,597 feet, or 1 1/4 miles (not counting some minor elevation gains and losses).