The Two Tunnels 50km is run at the same time as other distances (5km, 10km, half, full marathon) and is 5 laps of the 10km course. To minimise crowding and disruption, the other distances are released in waves, but the ultra runners set off over an hour before anyone else. 

The RD gave us a good race briefing. The marshals had an easier job of it than usual as it was an out and back course and it's hard to take a wrong turn in a giant tunnel, but they had a long day and cheered us on by name throughout. Relish Running volunteers all get a free entry so they're all runners and know what's up. These races aren't cheap so I'm not sure if they're value for money, but there's food, music, and drink at the start and finish and they're well organised.

Aside from other runners and the marshals the only local support is when we looped through the start and finish. There were plenty of people out enjoyed the cyclepath and they and the runners were good about sharing the path but there's always a few people angry about disruption to their stroll/cycle  and that causes a bit of a safety hazard, especially in the tunnels. The course is very unique, it's the UK's longest underground race, and it winds under some lovely trees and out into the countryside, but it's there's not much of a view.