There are people who do marathons with little experience, but the factor is called "base mileage" - the more base weekly mileage you have, the less marathon training will suck, because your ideal training plan starts where you are with a base and builds from there. it's not about ONE long run - from personal experience, my first marathon, my longest run was 22 miles but only peaked at about 35 miles per week, and the race was so bad that I would strongly urge anyone to have a higher base and try to peak at at least 40 miles per week.
That said, there are sensible programs that build up from where you are. Please follow a program. halhigdon.com/training/51143/Marathon-Novi
Finally, I strongly urge you to run a few races at shorter distances because a marathon is an awful time to discover that you hate racing.
Edited by: ZORBS13 at: 1/23/2012 (05:45)