From last year, you can just follow the pacers; they have a huge group and lots of balloons, they're easy to see.
I'm going ignore them this year though. Last year I thought I might be on for a time of around 4:30 and thought it might be a good idea to follow the pacers.
But I lost touch with them after I stopped for a (heh, heh) comfort break on the first return to the stadium. I them spent the next five miles hoping I'd catch them again, not realising that this had got me into the trap of going to fast.
I blew up badly and trailed in with a time of 5 hours - the pacers probably in the pub by that point.
So this year I'm going to stick to my own pacing, however slow it is!