I've just tested myself on my longest run before the Royal Parks HM in 2 weeks' time. I did 20:16km, which took me 2 hours 48 minutes.
I was so nervous before I set out, as my training schedule went out of the window when I injured my leg - but all went OK! My biggest problem was that my phone battery gave out about 20 mins before the end, so I lost my comforting little ZenLabs robot-voice - but I know I can go the distance now.
I was doing run/walk, but I was running most of the way - and I finally seem to have found a pace that works for me. Thank goodness!