A friend and I are going to attempt the Tube Challenge for the first time on Tuesday. We're still putting times to our attempted route - we're just doing it for fun, not to try and get a particularly good time. We were having issues finishing on time; starting from Chesham instead of Watford might have fixed that, it's hard to tell yet (as I said, I'm still putting times to it!). I might have to grovel for some last-minute route improvement tips if I still can't get it to work

. We're finishing at Heathrow.
Looking at the TfL website, the number 79 bus ends up taking 17 minutes when you put it in a journey (apart from at a few times of day when it inexplicably takes 5). However, according to
http://www.londonbusroutes.net/times/079.htm , they do indeed take five minutes. Is this because the journey planner assumes you arrive just after a bus leaves or something (ie the worst case)? Or is there something else odd going on?
Thanks!