If someone would like to be a Ultra Runner from zero, for how long would have to train for the body to get in perfect shape of an 80k ultra race... 6 months? 1 year? 2-3 years?
There are going to be a few different questions that will somewhat dictate the time needed to train.
- How much have you run in the past? - If you haven't run much, then getting to the point where you can do 50 miles (roughly 80k) is going to take longer than someone that has a more extensive running background. You also need to progress slower, as your body will be adapting to both the running as a new activity, and increasing distance. A more experienced runner will have some of those adaptations already.
- How consistent can you be in your training? - Running is one of those things where it isn't a few months of working out will get you everything you want, but long extended periods of putting in the work and the mileage day after day.
- How far can you run now? - If you can already easily run a 10k (slightly more than 6 miles), then you are further along and can shortcut some of the earlier build work.
- What is your goal for the race? - If your goal is just to finish it, then the time to train will be much less, and the type of training may be markedly different as well.
I have recommended this plan before, and the simple reason that I continue to do so is that it works. On many levels from recreational to highly competitive. 3-2-1. Take whatever distance is your longest achievable. For this example we will use a 10k run, and theoretically this is as far as you can currently run in one shot. To complete this run takes you 45 minutes. This would be the 1 of the 3-2-1. For the other runs, the 2 runs will be 30 minutes each, and the 3 runs will be 15 minutes each. Don't worry about pace too much, just run easy. Hard will come later, and as you get more fit, your pace will naturally increase of its own accord.
Structure it along the lines of short (15 min), medium (30 mins), short, medium, short, long (45 mins), day off. This gives you a starting point of about 33k or so for a week. You want to gradually build this up, usually a 5-10% increase in distance every couple weeks depending on training intensity, your own tolerance, things like that. This is where your own personal monitoring comes in, you should be able to feel if the last week left you beat up, or if you feel good enough to add a little extra training.
Initially you'll want to just concentrate on building up distance to where you could complete the 80k run. This may take a year, it may take 5, just depending on how you react to the training, and how much attention you pay to things like rest, recovery, nutrition, equipment, etc.
Once you are at the point where you can complete the distance, then you can start looking at adding in speedwork and other higher intensity workouts to be able to compete the race, rather than just complete it.
JohnP's answer is good.
To answer the question directly - probably 2 years would be safe - but it's kind of the wrong question.
Tim Knowles' book contains plans for a nice progression from no running at all through to finishing the ~90km Comrades Marathon. It starts by training for 10km, then a half, then the full marathon and then the ultra. Each stage is, from memory, about 4-6 months. That plan has be scientifically evaluated and shown to work pretty well and not have many injuries. Going faster than this is likely to cause problems.
I say that it's kind of the wrong question to ask. I believe people are ultrarunners because they enjoy running. They do it for a variety of reasons (and, if you were to dig deep enough, some of those reasons are about being able to brag) but they're more motivated by the joy of running and being part of that community.
So, instead of wondering how long it's going to take, I'd just get out there and start planning to run a 5km or 10km well.