Welcome lumbo. I'm mostly in the One gear camp but part of me is in the 'make sure you enjoy it' one.
You obviously have good speed over shorter distances but it doesn't always translate. I ran my first marathon at age 22 and finished in 3:09 but it was quite messy over the last 6 miles because I hadn't done anywhere near enough long stuff. Age is, of course, on your side so aiming high is not for criticism. Running can be a lifetime of enjoyment as many of us on here will testify but it can also be cut off at the pass with a bad early experience.
As TR often remarks actual MP doesn't have to be set in stone until nearer the time. I'm running my marathon in May and have an idea of what I would like to achieve but that idea can change 5 times on a single run!!!!
Enjoy the training, run to your capability and see what happens - that's what I say!
jools - is your decision made yet?
Anyhow, after yesterdays MLR I was sitting at my desk and got up to answer the door and twisted my knee. Was due to do a 5 mile RR this morning. Put my kit on but I could feel a twinge that bordered on pain. Got changed into a suit and came to work. Hopefully, will feel better tomorrow for my planned 12 miler. Feeling quite smug for resisting the temptation to 'try it out' and making it worse.