Running a marathon is a lofty goal. Take things one step at a time. Don't move one until you are really ready to. If you hit a spot where you think 10k runs are enough, then stay there for a while.
One mantra if "Respect the distance." I have run a coule of marathons and I discovered that 26 miles is not my cup of tea. I get thoroughly bored. So I am staying at Half marathon for a while. I plan on one full a year to see if I like it any better, but that's it.
Start by doing what is necessary, then do what is possible and suddenly you will be doing the impossible -- St Francis of Assisi
Rock 'n' Roll Dublin Half Marathon, Dublin, Ireland, 8/5/2013 ie.competitor.com/dublin/