Technically speaking, anything beyond the distance of a regular marathon (42.2km / 26.2 miles) is considered an ultra. The most common ultra distances are 50km, 100km and 100 miles. One of the main surprises for me, however, was that while most ordinary marathons are run on roads, almost no ultras are. There are some exceptions such as 24 hour races where you run as far as you can in 24 hours, often times on a loop. Similarly, "backyard ultras" where you run one lap of 4.167 miles (6.706 kilometers) every hour for up to 24 hours (hitting a total of 100 miles if you are able to keep going for the full duration) may be run on an even terrain. But the vast majority of ultras are run on trails.
These trails are oftentimes in the mountains. Thus, besides running the longer distance you also oftentimes add considerable vertical ascent to your runs. If you plan to run a trail ultra and have picked out your race, it is therefore vital that you add race-specific training such as hill repeats and long runs in relevant terrain to your weekly training schedule. As the demands on a hilly trail is much greater than that on the equivalent road marathon, you will burn far more calories on ultra trails than you are used to. The demand on your muscles and joints will also be greater, especially when running down steeper sections of a run.
Luckily there is a widely used formula to approximate the load of a trail race relative to a road race. You simply take the vertical ascent in meters and multiply is by 10x and then add it to the race distance. Example: If you are running a 50km trail race with 1700 meters of ascent, then the 1700 meters are considered to require a similar amount of energy as running an additional 17km (1700 x 10). The total comparable distance of this 50km race would therefore be 67km.
This makes the achievements of those able to complete iconic races such as the Hard Rock 100 or UTMB even more incredible. Hard Rock is 102.5 miles long, has a cumulative vertical gain of 33,197 feet of climb and 33,197 feet of descent for a total elevation change of 66,394 feet, and takes place at an average elevation of about 11,000 feet. The high point is 14,048 feet. The calculation is a little more complicated in feet as you will first need to convert those feet to miles. The 33,197 feet is approximately 6.3miles x 10 = 63miles of load. That's 102.5 miles of distance plus 63 miles of ascent equivalent = 163 miles of total load.
For the UTMB main event the distance is approximately 170km and the ascent approximately 10,000 meters, for a total load of 270km. But that's not it. If you're attending a high altitude race such as Hard Rock or UTMB you also need to take account of the altitude. Though there are many great races around sea level, so those would a better place to start.
Finally, be aware that course designers at times have a bit of an evil strain and may include river crossings or sections so steep you have to crawl on all fours. I've also participated in a race where there were hundreds of steps to climb up and then down again as part of the course. So it's important to keep a positive attitude, at times perhaps a sarcastic humour will be what's needed for you to succeed.
Gary Cantrell, legendary ultra-marathoner and creator of the grueling 100-mile trail race "The Barkley" in Tennessee's mountains, where only 15 of 1,000 runners since 1986 have finished within 60 hours, once said: “It never always gets worse.” In a similar vein Courtney Dauwalter, who holds the record for the longest distance ever run by a woman in the Big Dog Ultra (283.3 miles or 68 laps in 56 hours, 52 minutes, and 29 seconds) has coined the term "pain cave" to celebtrate the mental and physical state you need to accept to succeed in many races.
While these legends deserve all the credit what makes ultra running this truly amazing sport is its accessibility. In almost no other sport can you line up with your heros and participate in the same event. You don't have to run the entire race, in fact you don't have to run any of it. I've walked entire multi stage (250km) races. Race often times have cut off times around the 4km/hour (2.5 miles/hour) pace. Even seasoned ultra runners tend to walk the up-hill sections of ultras and only run the straights and downhills.
Because the ultra community is smaller and events often times involve some travel, I have noted that the participants are incredibly welcoming and approachable. I've had a race winner come up to me and explain the pros and cons of using poles in races (that was my first race and I didn't have any poles as I erroneously thought only tourists carry poles). I now always carry poles, especially when you get tired they help stabilize your legs and take pressure off your knees.
To date I've only participated in four ultra's but I've already made numerous friends. The community is so small that it's quite likely that you will meet familiar faces at different races. And since you'll probably run your races at similar paces it's quite likely that you'll run alongside the same people at different paces. I made friends in a race in Georgia whom I later traveled to a race in Kenya with. And I ran alongside an American runner in Georgia that I later met during a race in the Saharan desert. And then one of the runners whom continually leapfrogged me in the Sahara race suddenly appeared in the Kenyan race.
One final difference between regular marathons and ultra trail runs is that due to the remoteness of the routes and the oftentimes rapidly changing weather conditions, organizers require participants to run with vests or backpacks. The shear quantity of nutrition and drinks that will be needed will in anyway make running with a vest indispensable. The same applies to your long runs.
To conclude: Practice in a race-specific manner, prepare to eat and drink more, take it slower, walk the uphills. Have fun.