David Von Stroh submitted this ~50km route:

Run the Golden Gate Bridge and the Richmond Bridge in one go, starting from Embarcadero BART and finishing at El Cerrito del Norte BART in the clockwise direction (or reverse).

The Gotthard 100k navigates the heart of the Swiss Alps.
This is a fundamental road circuit, usually favored by motorsports enthusiasts and cyclists. However, it is also a perfect road ultra marathon route, that adds up to 100 km distance if start/finish are at the same spot. The route passes by snow-capped mountain peaks, glaciers and goes through three cantons (Uri, Valais, Ticino).

Submitted by Jeff Garmire: