@nomadic et al

The speed that they use for that website (apparently) was calculated using the TfL journey planner speeds which are probably not over exerting. I don't think that anyone would actually cycle to London Bridge in 12 minutes (and I don't even think you can manage that on the train), though if you average out the wait time between trains and take into account walking from your house to the station and the station to your destination you'll probably find it takes about 30 mins which I'd imagine is an average cycling time. When I used to work in London Bridge it would take me around 25mins.