as one of those interlopers to ultras referred to by lirish up there, take what i say with a large pinch of salt ... i think it depends on what you guys are used to running regularly. if you normally run 10ks as your long runs and the 21k races are big efforts then your legs probably will take a while to get used to the longer distances and build up. If you are used to running regular long runs of 20k +, and maybe even have some history of running further than this then you might be ok. have any of you ever run more than 30k??
You can train for a marathon in 3 months, but you wont be running fast, you will need walk breaks i expect, it will hurt and it will hurt afterwards. 50k isntmuch more than a marathon (says the girl who has never run-raced further than a marathon )
so i would say, maybe, depends and welcome to the world of considering an ultra...