I will ask the obvious question, why choose a marathon if you get bored running for over an hour? You will be needing to do quite a few runs over two hours to get the conditioning/endurance needed - I am a fairly low mileage runner and do a long run and medium long run weekly (they probably total 4 hours.), one tempo run of about an hour, and hill reps again about an hour. In addition to that I do a lot of cross training (rowing) hours. Where possible I will try to put in some recovery runs if possible. To make progress I consider this very lightweight compared to most pushing the 3hour mark.
I also need to ask how you know your endurance is fine. Endurance from cycling, rowing, swimming etc won't translate directly to running. I would say your speed is fine based on what you said about a 5k time, and endurance will need developing.
I'm not wanting to put you off - just to ask the questions before you dive in. Maybe look at SHADES, Hal Higdon, and Runners Word plans as a starting point
Alternatively, What about targeting some 10k races instead.
Edited: 26/01/2013 at 23:36