Check out Trail Running Mag , they have a list of runs.
It all depends on what type of running you wish to do. Ultras come in flat trails like the Thames path ultra and the hilly ones like the lakeland 50 and 100.
Personally i would far rather run on rough terrain as you avoid rep stress injury. There are also multi loop races where you run as far as you wish.
Ultras also come as marked trails and self navigation. Don,t underestimate how much navigating on the hoof will slow you down. Navigation can add to the fun of a race if all goes well and be the pits of not. Most races allow the use of a GPS but you have to be able to use one without thinking. ( on longer races you will run out of brain power, even trying to open a gate can become mental strain!)
The good news is that the ethos of ultra running is superb . You can forget PB,s but expect help from all the runners around you.
One much overlooked source of ultras is the LDWA. Runners are more than welcome and these events are cheep ,well supported with superb food at multiple checkpoints.
As for training you need running time One way to add the miles is to run shorter races twice . Start your race in the small hours(preferably with a fellow ultra nut) and plan to arrive back at the start just before kick off for the main event. The other runners will drag you along to the finish pushing you harder as you will be in race mode.
Edited: 13/11/2011 at 18:10