The Rough Trail traverses the heart of the Red River Gorge from west to east.  You get tons of stunning views of the Red River Gorge as well as plenty of creek/river crossings, and of course lots of climbing.  This route is a staple to the area, and there is a reason its called the Rough Trail.  The terrain is unforgiving, be prepared to watch your feet on every decent to avoid a costly step on a rock which could lead you to falling.  This FKT route is an out and back, seeing all sites two times along the way.

Vildsmarksleden in Bjursås Sweden is a very technical challenge with the majority of the track located in the forest with tree roots and stones. But it also passes a couple of hill farmes which makes it a culture link to the old, harse, way of living in the country side. The terrain is not flat leading to numerous nice views along the track.

https://www.visitdalarna.se/gora/aktiviteter/vildmarksleden-bjursas-35-…

A route that is simple, but difficult. Go to the Forest Hill Park in Richmond, VA and run the hill 100 times.