The first step to choosing that first ultra is actually choosing the race. The single best resource is Ultrarunning Magazine's online ultra calendar (http://www.ultrarunning.com/calendar.html). The races can be sorted in a variety of ways based on location, distance, difficulty, date, etc. I would recommend starting with distance. There's actually to categories: distance races and timed races. Here are the most common:
- 50k (31 miles): This is considered the minimum “ultra” distance. It's usually considered to be the easiest. It's a good first choice, especially if you don't want to train for a long period of time.
- 50 miles: This is also a decent choice for a first ultra. It's more difficult, but can still be run in a single day.
- 100k: This distance is getting into the realm of “too difficult for a first ultra” because they usually require day and night running.
- 100 miles: Some people start with ultras, but I'd advise against it. You encounter a lot of issues that are difficult to experience in training. Your best bet is to learn from the shorter distances first.
- 6 hour race: This is probably the shortest timed race where a beginner could reach the ultra distance (50k). The races are usually held on small loop courses with ample aid and other support. However, it would probably require running for the entire 6 hours averaging about an 11:30 minute/mile pace. It is probably better to do a...
- 12 hour race: This is a great ultra distance to get your feet wet if a 50k distance race is unavailable. Pretty much anybody can reach the ultra distance and still have plenty of time to sit down and rest, eat, and take care of any issues that arise. Think of it a an ultra with training wheels for the newbie. In fact, 12 hours are my favorite race to introduce people to ultra distances by using simple logic- you can stop at any time, even after one lap.
- 24 hour race: This is another good option if a 12 hour isn't available. It can also be a good option for those wanting to train for 100 milers since it involves night running and sleep deprivation.
- 48 and 72 hour races: This would be overkill for a newbie... no need to go this long unless you're really into adventure!
As if this weren't enough options, races also come in a few different layouts:
- Point-to-point: This race starts at one place and ends at another. Timed races do not use this format. Personally this is my favorite type of race because you don't run on the same part of the course twice. I like the novelty. For beginners, this is a good choice because you don't come back to the start line, which could give you the opportunity to drop out of the race (DNF, or Did Not Finish).
- Loop: A loop course is run in a series of, well, loops. All timed races are loop races. Many distance races run in smaller parks are also loop course. The advantage of loops is course familiarity. Once you run one loop, you've seen all of the course. For some, this is a nice benefit.
- Out-and-back: These races start at one point, run to another, then back to the original start line. They have the advantage of course familiarity on the way back, but also make it difficult to stop. Unless you have a rise to the start line, dropping out would still require you to walk all the way back to the start line- the same distance you'd cover if you stayed in the race. Personally I like to plan training runs as out-and-backs for this very reason.
- Stage Races: These races are held over the course of a few days. You run a certain distance, rest, then run more the next day. Because of the logistics, these really aren't good "newbie' races... and some question if they're actually ultramarathons if the runners don't cover at least 50k per day.