The London marathon is not the only mass participation event in April. In Vancouver there is the world's largest 10k that attracts around 50,000 people. This year it took place on the 19th and not only did my sister's family take part, they excelled.
My sister finished in 51.46 and was 4864, my brother in law managed 45.22 and was 1573, my niece, on virtually no training, managed 59.35, and my brother-in-law's nephew, who doesn't run but lifts weights for exercise, finished in 1.13.43. Now I think all these results are brilliant and some become even better when adjusted for age (my sister finished 17th in her age group and my brother-in-law was 68th). However there is magic in round numbers and the target of breaking 50 minutes and 45 minutes was not quite achieved (in one case by only 23 seconds) so satisfaction was not complete. The quest has to go on and they are now looking for other races in which to break their barriers.
I am sure they will find the right race as they are very close to their targets, especially as it is difficult to run your best in a very crowded race. My biggest race was the Great North Run and my memory is of weaving around other runners, continually slowing down and speeding up so that it was very difficult to maintain a rhythm and build a race pace. I am sure a smaller race, with enough people for an atmosphere but clear space to run in, is a better bet for a PB.
P.S. A list of the largest races is here. In 2008, the Vancouver Sun Run wass 4th, the Great North Run 10th and London was 12th. It will be interesting to see how the economic crash will effect the numbers in 2009. We might run more because it is a relatively cheap and can help us forget about problems but we might enter fewer races because of the cost. Who knows.
P.P.S. I like the names people invent for races and am particularly taken by the Lilac Bloomsday Run. Now I love any run which gives a hat tip to the wanderings of Leopold Bloom but it is run in May when the official Bloomsday is 16th June. I'm sure there must be some good reason for the discrepancy but I have no idea what it is.