If you can't run past 2k then that is a general running question rather than an ultra question... however, for me (and obviously my experience only) distance running came in two phases.
I got into running because I got bullied into entering an Ironman when I'd never run more than 5k before. I spent a several years trying to build up the distance at a reasonable rate and dragging myself around marathon courses but was persistently dogged by knee injuries and mounting physio bills. FOR ME (deliberately shouty) the solution was a deliberate change to my running gate and footwear. From heel striking and various models of 'stability control' running shoe to mid/forefoot strike with semi-minimal/low heel drop shoes.
I don't really want to spark a big debate about fore/mid/heel strike, to me zealots at either end of the spectrum damage the discussion, but it worked for me. One thing I would say is that it took me about 2 years of deliberately transitioning through several different shoes and being super cautious about changing too quickly (e.g. I would often run 5k in normal shoes then switch to vibrams for the final 2k, and then gradually increase the distance). Still, I would say that I was mostly adapted to my new gait by about 2011 and other than a few niggles I've been essentially injury free since then.
WRT building up from IM/marathon distance to ultras, my view is to build up slowly. I simply got into a habit of running the following programme:
Wednesday: start at 10k build gradually to 22k
Weekend: back to back long runs starting at about 30k for the weekend and building towards about 85km over the weekend (split however you like but I liked 42/42 or 50/35).
Build the distance each week but listen to your body and build up in small increments. Take every 4th week off. In rest weeks I think I usually ran most days but at a very easy pace and never over 10k a day.
My first attempt at a 100miler I used this schedule over about 6months and dnf'd, although I did make it ~75miles and had done the first 50 in about 8 hours (waaaay faster than planned). My second attempt I probably ran a similar weekly volume in training but I worked less hard during each run and built up over 9 months instead.
All of this advice goes out of the window soon, as I'm currently averaging under 20km/week for 2016 and I have a marathon planned for October and then probably a 50 in spring... I'm hoping my body just remembers how to do it :-)