Why not? Someone wanted to quantify "slightly faster than average", so I made up some percentiles. I don't think there is a scientific definition of "slightly faster than average" so I think I can do this. You could I suppose counter with you think it means "35th-49th percentiles" which is valid but doesn't really change the argument too much.
> Do we know the distribution of first-and-only marathon runners to repeat runners or is this a pure intuition?
Just intuition. It would be a tricky thing to survey, as you would need to wait for all current people who have ran exactly 1 marathon to die, to confirm they do not indeed run more .
I did look for those stats for London but I couldn't find anything relevant - I'd assume they collect that information on the entry form but it's possible they don't or just don't care to publicise it later.
> The average finishing time globally for 26.2 miles in 2014 was 4hr 21min 21sec – about 40 seconds faster than the average for the period 2009-2013. Men’s average finishing time was 4hr 13min 23sec, while women’s was 4h 42min 33sec – 29min 10sec slower.
I'd suggest your 3:30 was still definitely more than "slightly above average".
If you want to quibble about the median instead of the mean,
e.g. http://www.nytimes.com/2009/10/23/sports/23marathon.html
> In 2008, the median finishing time [for men] was 4:16, a pace of 9:46.
Which still puts 3:30 way above, I think.
I make that a 3:30 marathon which I would suggest is rather more than "slightly above average" - you'd be in the first 25% male finishers at London 2015, for example.