I did a session of the very-reliable Yasso 800s today that predicted my marathon finish time, at my current level of fitness (i already have the endurance base thanks to work) as about 3 hours!!
Clearly, however, for my first ever actual marathon I will NOT be trying to be some "PB hero" and am still going to stick to the more conservative (and fun?) 4 hour target. Specially as it's London - unless you're an elite athlete, big races like London are an experience to be savoured, not a dull race to set PBs in! (just my opinion :) )