Hi, I'm not a first timer, and it's not even London I'm doing ,- Lochaber instead- also in april.
I love these "first timer" threads, but having lokked at few similar ones, can I make some suggestions:
Consider walk/ running both in traiing AND in the race as a planned thing, rather than gettingn forced into it. Some people feel it is a sign of failure, but look up Jeff Galloway- he advocates it for even very good runners. I succumbed to ITBS on my first campaign, and this approach saved the day ( 4hr 20 mins, with a 30sec- 1 min walk at every mile marker)
Make sure you do your LSR's slow enough- get a HR monitor. Do a max HR test, and keep down to 70-75% for these, it's very slow, but it's the best way to get the distances increased.
Don't plan your finishing time at thei stage, just do the trainig, then do a half marathon a few weeks before the race, use the comversion tables available on the internet to calculate your probable time, add a bit (they tend to be optimistic), and set off with the nearest available pacer if you can find one. (They have pacing groups for VLM- great idea)
If it's not too late already, start your trainig plan 1-2 weeks early, to allow for inevitable missed runs due to life getting in the way- and DON'T try to do xtra to make up for missed runs when the inevitable happens.............that way lies injury.
Good luck.
Enjoy