Hi - I ran the LM a couple of years ago and I have to say it was one of the most wonderful experiences of my life. I had never run before but got drunk one night and agreed to do it for a charity that we had close ties with.
Tips - stay in a hotel the night before with a coach to the start - otherwise you end up walking an additional few miles before the start. Or see if your local running club has a coach going up and hitch a lift
Go to loo at least 3 times before you start - the first set of loos on the course had a huge queue right across the road!
Go off slowly - everyone speeds off, but you pass them is a few miles when they have started to walk because they were too quick
Target things to pass / keep up with if you are running - I can pass the large mince pies / Mr Men / trees / rhino/ man with hod of bricks.
Take the time to high five the kids on the route - if you can, stop and have a little dance in front of the bands - you are not going to win (unless this is Paula R in disguise) so take your time, enjoy it.
Have an absoloutely wonderful day - I still get shivers watching it on TV (though not sure I could do all the training again) - and smile and wave as you come over the finish line so you can buy the lovely photo of you achieving something a tiny minority of people achieve (making myself tear up now!!!)
Good luck - oh - and I must mention you will go completelt loopy the week or so before hand - this apparently is quite normal - just remember buying a £30 pair of socks will not really help you!