Rides for overweight unfit people?

No, what you need is short high intensity intervals. Much more effective per unit time if losing weight is the objective. Lots of anecdote and even some evidence points to this http://www.google.co.uk/search?q=HIIT+weight+loss

Long steady rides are nice for enjoying cycling and the countryside, but for getting fit and losing lard HIIT is the way to go.

mdcc, got any good links to sessions that you like / recommend? I quite like a bit of interval training, but there are plenty of differing opinions about interval and rest leength and length of session etc. For example, unless on a turbo or exercise bike, tabata intervals of 20 sec isn't that managable. However, rowing ergs, elipticals, lend themselves to controlled interval training really well.