Twenty six.... or more?

Watching the start of the London Marathon yesterday, seeing the amazing sight of the thousands of people running for charity I got to thinking.......

.....Surely, a lot of those runners actually do far more than the 26 miles of the marathon, after all, only a few of them start off on the actual start line, those further back have a much longer to run, as they all have to go to the same finishing line.

I wonder what distance those at the very back of the crowd that set off actually cover??

Perhaps someone who has run it can tell me the answer to that?