@lazi: training, at least the way I do it, begins at 15 kilometres, being able to do it at any given moment. I gradually take it up to about 25-30 km multiple days in a row. At that stage it's not about the distance but about the time spent doing it. Doing this, in 3 to 4 months I'm ready for, say, 'de Nijmeegse Wandelvierdaagse' (the 4-day Walk of Nijmegen), 50 km (age specific) every day, for 4 days, time limited.
It's not everybody's cookie.