Sorry in advance for the long rambling post............
I've not been running long so I'm enjoying the fact PB's are easy to achieve just now. However it does mean that I've no idea of how quick I should be thinking of trying to do London in.
I've ran one marathon. It was Edinburgh in May this year and I ran it in a conservative 3hr 59min. (I hadn't really trained for it, well not much running at least)
Since then I've mostly been running Parkruns and have reduced my PB from 25min to 19min 23sec.
During this period I've also ran the Great Scottish Run in 1hr 39min and a local 10k in 41min 02sec.
Then this morning I went for a long run that turned into a bit of tempo run and I set a new PB for a half. It was 1hr 38min (55min for the 1st half and then 43 for the 2nd) with an overall avg HR of 139.
I've used the McMillan Pace Calculator but I'm wary of basing my marathon training on a 5k time especially since I don't really have many miles of training under my belt. (McMillan suggests a marathon time of 3hr 09min based on my 5k but to be honest I find that a bit scary)
Should I just find a half marathon, see how quick I can do it, and then pace my training on that time? Or should I be basing my training on heart rate?
Again apologies for the rambling nature of this post....my copy of P&D hasn't even arrived yet so prepare yourselves for even more tedious questions when it does. 
Edited: 11/11/2012 at 15:29