Try looking at the Northumberland Fell Runners site and maybe do one or two of the race routes. In the 'What's on section, the race details are often accompanied by route maps.

http://www.northumberlandfellrunners.../whats_on.html

A 20 miler would be the Chevy Chase (starting from Wooler). Shorter routes from Alwinton or nearby include the wonderful Windy Gyle race - which would probably come close to the 20 by the time you'd run from Alwinton to the start at Barrowburn. And obviously the Alwinton Round.

Enjoy