A brief guide to ultramarathon distances
There are a variety of ultramarathon distances, each with its own 'flavor'.
In many ways, the 50K is an odd distance. It is only about 5 miles longer than a marathon, rather than part of the usual doubling that occurs in the progression of '5K-10K-half marathon-marathon-50 miles-100 miles'. Some runners can run a 50K like a marathon - continuous running at an intense pace, where every second counts. For most runners however, this approach does not work, and the 50K needs be treated as an ultramarathon, with varying the intensity, eating, etc.
The 50K can be used as "over distance training" for a marathon. Running a 50K at a slow and easy pace builds endurance and confidence for a marathon. There is a psychological barrier for many marathon runners about the marathon distance. Going further can help overcome this barrier and promote better marathon performance.
A 50K can be a good choice as a first ultramarathon as long as it is treated like an ultramarathon, not a marathon. I believe that a sub 4:30 marathon runner can complete a 50K using a run/walk pattern without excessive training. A run/walk pattern of 15 minutes running with 3 minutes walking seems to work well.
2 50 Miles
This is a sweet distance. It is far enough to be a serious challenge, but is achievable with suitable training. In many ways it reminds me of the half marathon; The half marathon is an endurance race without the difficulty and long training require for the marathon. Likewise, the 50 miles is an ultramarathon, but without the problems of the 100 mile races. A 50 mile race requires respect, but a runner with the dedication to focus on the training required can finish the 50 mile distance.
A 50 mile race requires significant adaptation from the marathon distance. Very few runners can complete a 50 mile race without some walking and eating solid food. The 50 mile distance also generally requires significantly more training time than the marathon.
Races of 40 miles are broadly similar to the 50 mile distance. Some of them have some extra challenges to make them a very similar level of difficulty to the 50 miles. Likewise, the 100K (62 miles) is similar enough to the 50 mile to be considered in the same broad category.
3 100 Miles
Just as the marathon is more than twice as hard as the half marathon, so the 100 mile race is more than twice as hard as the 50 mile race. For many runners, the 100 mile race is summed up in the saying "You won't die, but you might wish you had." Just look at the DNF (Did Not Finish) rate for 100 mile races and you will get a sense of how difficult this distance is. I know of good, dedicated runners who failed many times at the 100 mile distance before succeeding. Finishing a 100 mile race is in some ways similar to qualifying for the Boston Marathon; while there are a few runners who can do it relatively easily, for most it is a herculean effort requiring multiple attempts.
The 100 mile distance brings new challenges to runners that are not seen at shorter distances. For instance, sleep deprivation becomes an issue for most runners, as they are running over night, sometime for more than 30 hours. Running at night can be a psychological challenge as well as a practical one.
4 24 Hours
A timed race, like 24 hours, is obviously different to a distance race. With a distance race, the slower you go, the longer it takes. This tends to punish slower runners even more, as they are on their feet for longer. With a timed race, the slower you go, the easier it is. If you reduce your pace, the lower the intensity AND the shorter the distance. Of course, the opposite is true - when you push things hard, the race becomes disproportionately harder. Timed races are normally run on a small loop course, which make them sociable events. You get to see, and often run with, the fastest and slowest runners. This type of loop course also makes logistics simple; your gear is never far away. The opportunity to take a break is always available, which is a two edged sword. A few runners will keep going continually, but most will take some rests. This might be 5-10 minutes here and there, or it might be an 8 hour sleep. I have even known people go off to a wedding and come back! Overall, a 24 hour race is a wonderful opportunity, regardless of if you are a complete beginner, or an elite ultrarunner. A 24 hour race can also be used in different ways, especially for a cheap race like Hinson Lake 24 Hour (HL24) which is only $24.
4.1 Marathon preparation race
For some, a cheap 24 hour race, such as HL24, is the perfect marathon tune up race. You can set whatever distance you like, but 18-24 miles would be typical. HL24 provides the real race feel, which is hard to replicate in training, combined with outstanding aid. The course has a near perfect trail for the distance, being soft, wide dirt with a few boardwalks, and almost flat. While this might be one of the few places you hear "I'm only doing 20 today", no one think any less of someone doing that distance. The loop course makes it easy to bail early if things go badly, which is sometimes a possibility with a marathon preparation race. To add sugar, the low entry fee also makes HL24 a very cost effective marathon preparation race.
4.2 Fixed distance
For another group of runners, a 24 hour race represents the chance to cover a specific distance without any time limits cutting in, and under easier conditions than most races. Common distances I heard people doing were standard marathon (26.2 miles), 50 miles and 100K (62 miles). Some were aiming for 100 miles, but at that distance, the time limit does become a factor. I saw one couple at the 2009 Hinson Lake 24 Hour push on through the night to make 100K, visibly battling exhaustion together; I thought it was very romantic. I've also seen the US 50K age record being broken at the 2010 Freedom Park 24 Hour.
4.3 How far can you go?
If you want to know "how far can I go?" a 24 hour race is an ideal place to find out. Eventually the reasons to stop become greater than the reasons to keep going, and movement ceases. Different people will stop for different reasons, but they can all push the boundaries of their mental and physical capabilities. ("Mental fortitude is more important than physical endurance", but that's another blog entry!) Some people will go until they can't go any more and stop. Others will take breaks to increase their distance, some for a few minutes, some lying down to sleep for a few hours. This makes a 24 hour race a great place for a first time ultra runner. There is no fixed distance that has to be conquered, no possibility of 'failure'. My friend Vince did his first ultra at HL24 this year and covered an outstanding 100K. In his own words "This was a fun race, kind of the gateway drug to Ultras. There were people of all ages (7 to 72) and abilities, each with different goals. Everyone was encouraging and friendly and there was no pressure."
4.4 Fun with friends
The loop nature of a 24 hour race means that you get so see nearly everyone at some point. The front runners and the slowest all share the trail as they go around the loop. This makes a 24 hour race a supremely sociable race. In other ultras you can be on your own for hours at a time, but not a 24 hour race. Seeing other runners, checking on their progress, giving and receiving encouragement is a great part of the race. For some, the social side is an important aspect. Friends you make in the middle of the night while you are both suffering through an endurance event are not like other friends; the bond is different, and I won't even attempt to explain further ;}
4.5 Racing 24 hours
For a few, there is the prospect of pushing the boundaries of the 24 hour limit and cranking out lots of miles.
I have not participated in multi day events, so I cannot provide any useful advice. One day I hope to update this section with some personal experience ;}