Bristol Bridgewalk
I regularly teach the Königsberg bridge problem and the modern algorithms that followed. While teaching at the University of Bristol I noticed that, like Königsberg, Bristol is built around two islands in a river. After doing some maths, and more exploratory trips, I found that the bridgewalk is possible in Bristol: One can go on a walk that crosses very bridge exactly once (at least every one that would have been included in the original puzzle). Having found the solution I naturally had to walk it. The first bridgewalk took 11 hours and was about 53km long.
Since my orginial bridgewalk in 2011 some new bridges have been built, but this has actually made the walk slightly shorter. The map and instructions on this website describe this updated walk. I now typically walk it in about 9 hours. The bridgewalk follows a very pleasant route that goes past some of the most beautiful spots in Bristol. It mostly avoids traffic, following footpaths and smaller streets along the Avon and Bristol harbour. It naturally breaks into two halves which by themselve make relatively easy walks.