Training on a Brompton

I have been toying with the notion of participating in the 'Nightrider' 100km cycle ride in the summer. This is a charity bike ride where one takes in many of the sights of London at night. I'd love to do it but feel I am not fit enough or have enough miles under my belt.

Tonight I decided to go out on my first training run. I kept things very simple. I cycled round the block which is quiet and fairly car free for 20 minutes. I kept up a brisk pace and was out of my saddle for a fairly sustained incline.

I would love to keep this up and after a week or so increase the time I cycle slightly. Any advice on how often to do this would be welcome.