In reply to David Robinson:
As Iain says, with a 3:45 marathon, you'll probably be fine.
For races to start with, literally anything (as long as its not a race catagorised as AL) would probably work - but are location dependent, unless you like a nice long drive to get to a 2 mile race.
Perhaps the most important point is, can you navigate? As that is pretty fundamental, even if it is a relatively straightforward race.
To give you an idea, when I started, I barely ran at all, and just thought that I needed to lose weight for climbing. What can I do?
Entered an 8 hour adventure race - they kind that goes from a hub (questars or similar), something I could can at any time, and then just went for it.
Others may disagree, but maybe a Rab mini mountain marathon type thing might be good. Ok, its not an out and out fell race, but its off road, its navigational, its timed, and if you don't think you're up to it at any point, you can traipse back to the start. Also, there isn't a pack mentality, so you won't need to be worried about not "keeping up"
Once you've done that and realised its not all that scary, enter a local race, and go for it.