To give yourself a good shot at a first-time marathon, you should probably be looking to do one no sooner than next autumn. It's possible to do one sooner, but that would mean increasing your distances more quickly than is ideal, resulting in a higher risk of injury. And unfortunately, those of us who start running when we're a bit over 21 are more likely than the youngsters to get silly overuse injuries.
The long run is definitely the most important part of marathon training - nothing else can prepare you for the grind of being out there giving your all for several hours.
There are lots of guides, in print and on the internet, to marathon training. Gobble them up! Also find out if there is anyone at your club who can advise you or, better still, train with you.
Happy running!
Cheers, V-rap.
PS. This is "do as I say, not as I do". I'm not as fast as you and spent most of the early part of this year mollycoddling an overuse injury, but my form for London 2003 has been sent off.