Trail would be better as a halfway house between road and fell. It depends where Sam lives.
As an already reasonably quick half marathon runner, though, I reckon he could get up to 20 miles on the road in about 4 to 6 weeks (say 2 hours 30). That would be a great jumping off point for a transition to long fell races (and also to appreciate the difference between 20 miles road and 20 miles fell).
Also take a look at LDWA challenge events, many of which welcome runners. Cheap as chips, food provided, chance to navigate, learn to carry kit, pleasant scenery and excellent traning value.
http://www.ldwa.org.uk/challenge_eve...L&event_type=N