I started with Cader Idris as well. I reckon it was pretty good for a first race.

If you are coming from a road running background I would look for an out and back race or something well flagged as it limits the navigation needed. I think that the need to navigate and run is much more of a worry than the hills.

How long or hard a race you choose is really up to you, if you are fit I don't think there is any reason not to start with something hard as long as you are safe.

Jonathan