There's an interesting article on the BBC News site about the circuit ultra-run this weekend in Gloucester.
Circuit ultra-running is daunting - rather than running a long course, it's just laps of a standard 400m track.
The changing scenery and sense of achievement I get from running long distances? Nope, none of that for circuit ultra-runners, just endless laps.
There's much bigger mental challenge in circuits. When I get tired 10 miles from home, I know that the only way to finish is to keep going.
On a circuit, rest is never more than 100m away. The grit to keep going is far more abstract, far more about personal discipline than about necessity.