To a great extent there is no point in doing any work on your starting technique in large mass participation road races as it is insignificant.
What is important, however, is how you run once you get going. This is often determined by the pace of the people around you, so it is vital to ensure you are starting with people who will run at the pace you want to run at.
In races like the London Marathon or Great North Run you will be given guidelines as to the quality of the people around you, so ensure you stick to your plan and get in the right group. Starting too fast is always a big mistake and can make the second half of any race extremely uncomfortable.
David Coleman, one of the top commentators on athletics of all-time, used to commentate on the London Marathon every year. As the race was starting he would almost invariably give out the advice that had once been given to him.
"Start slowly and slow down."
Although a little pessimistic, many a marathon has been successfully run by athletes new to the event, by taking this advice to heart.