Well actually it was 9.6K - the whole thing was a little unintentional... meant to do about 6 miles but didn't realise that translated to 10K. Anyway, it wasn't quite as hard as I thought it was going to be, but a nice ambient outside temperate of 15C helped as well as the beautiful scenery by the Thames in Rotherhithe. But most of all, the sense of achievement afterwards has been fantastic. Definitely plan on doing that again.