Started running 3 Jan 2013 and did first marathon at Portsmouth in 22nd December.
As has been said before, it's whatever feels right for you, only you'll know if you are ready. For me, doing a 13 mile run on a Friday followed by a 20 on the Sunday in early November suggested (to me) that I could do it.
I was less prepared for my second marathon (6 April) and now have a niggling IT Band knee issue. Hoe hum!
Simonp83, Can't recommend a runner's watch highly enough. I used to use my phone too, but the hassle of start/stopping accurately was a real pain. The last straw was failing and landing on it. The screen didn't take it to well.
As for training plans, personally I can't seem to find something that I can gel with, as a shift worker, my nights and days can be a little screwy.
My first marathon last month was done with plenty of LSRs and over confidence. Having said that, I got around Portsmouth to plan in 4:22 so was very pleased.
Attempting to repeat that performance in early April, then up my game for a sub 4 in Edinburgh.