I guess it is cheating to keep using articles I copy from Ultrarunner, but they are good, I give proper credit and things are really busy right now so here is another.
An article from Ultrarunner Magazine.
Times have changed and now we see a great variety of choices in each month's listings. Since no one can do them all, and most of us can do very few, a selective process is necessary.The first factor to consider is distance. Weigh this factor based on your experience. If you have never even run a marathon, then do one first. Certainly many, perhaps most, of us can successfully negotiate an ultramarathon without prior experience at the marathon distance, but that is not the point. Ultrarunning reserves its greatest rewards for those with the patience to work toward long-term goals. The first lesson that we each must learn is how to take one step at a time; that is how every ultra is done. Marathoners deal with a mythical 20-mile wall. For ultrarunners there turns out to be a series of walls, each indicating a change in the basic nature of the race in question. If we bypass all these landmarks and run a 1,000-mile race after our first 10-km, then we have wasted the opportunity of enjoying the personal fulfillment at the successful passing of each of these barriers.
- Races under 20 miles are your basic road races. Be it a 10-km or a 30-km, the factors to be reckoned with are roughly the same. Being able to finish is not the question; it is simply a matter of how fast.
- The 20-40 mile distance consists, essentially, of races similar to a marathon. Fifty kilometers is technically an ultra, but it is run simply as a long marathon. At these distances mistakes no longer penalize only your finishing time, but bring to the fore the very real possibility of failure to finish at all. The 20-mile wall is real, and going beyond it while attempting to perform at the maximum of your ability is an accomplishment to be proud of.
- The range between 40 and 70 miles brings us to the realm of the 50-mile and 100-km. The barrier we passed at 20 miles seems only to have been put there to prepare us for the bigger wall waiting between 40 and 45 miles. For the average runner, walking is now an important part of the equation for success. Still, these are essentially running events.
- Races between 75 and 100 miles put us into elite company. Walking is now a major consideration and sleep deprivation becomes a new critical factor. If the barrier we conquered to reach 50 miles seemed demoralizing, the wall between that and 100 miles is devastating beyond description. Training and experience may render marathons and 50-milers routine, but even the great ultrarunners will tell you that 100 miles is always hard.
- At 120 miles and beyond we reach the multi-day level (if you can run 120+ miles in an event that is not a multi-day, then my advice will be of no use to you anyway). At these distances the barriers are no longer clearly defined and periods of depression and elation rise and fall as inevitably as the ocean's tides. Here, during these ultimate running experiences, we one day reach the realization that no longer are we limited by distance, but only by the time it will take to achieve it.
So my first sage advice is to take each of these steps one at a time. Savor each moment of success, celebrate each passage into greater things separately, and, most of all, learn to appreciate the journey as well as its completion.
Later, as the distances become part of your normal range, you can go after the challenges of the monster courses and the celebrations of the big races. Some will become annual pilgrimages and others you'll taste once only as you move on in the quest for new experiences. These decisions may not be so much conscious decisions as simply a feeling you'll have in your heart about certain events.
There is one final consideration in picking an ultra: location. Initially you might prefer to stay close to home and concentrate on the race itself. As he or she matures, the smart ultrarunner begins to think about more exotic locales. Ultrarunning constitutes more than just an opportunity to travel; it is a reason to travel. The average tourist visits a place by staying in a motel full of tourists, visiting tourist places, and, generally, leaving without ever really "seeing" the place at all. As an ultrarunner we go and spend our time with the local runners, doing something that gives us a genuine taste of the locales we visit.