A personal trainer focuses your mind, and body, on exercising parts of your body that you might forget about, even if you religiously do floor exercises while travelling away from your normal life.
Also in London, Matt Roberts has a big following. He used to be based at One Aldwych and also at the Royal Garden Hotel.
Whenever he has a moment he seems to head over to 10 Downing Street, the official home of the Prime Minister, and yes, Matt Roberts has been seen taking D. Cameron out running.
So, when in Rome, or rather London, do as the Prime Minister does, and use a personal trainer.