No way. I used to do running (it doesn't transfer at all). In my opinion a marathon distance would be more like 200km in the cycling world or Noodley might be right (I haven't done anything like 400km and probably never will). There are two reasons for this, firstly total newbies come on here all the time and do the sort of 100km from scratch in a few months and secondly, I a very mediocre and slow cyclist have been doing about 2x100km rides a month. If you were better than I am and have the time you could conceivably do them every weekend no problems. Marathons aren't the same.