Do plan to motivate the obese to buy a bike and start riding to work? OR are you planning on leading a bi weekly 5 miler? It seems that knowing your eating issue this is what you bring to the table?
If I was starting it I would seriously consider
buying investing in some bikes and various helmets and try and get people hooked on cycling, it would give you in my opinion a serious USP in the current niche market. Most people over weight or not, are Gym shy it would be a perfect solution. I got back in to cycling purely by chance in the summer of 2009 holidaying in France when my kids kept on and on about hiring some bikes, within 3 days I was hooked. Within 20 months I am 3 stone lighter.
I can picture the quarter page adverts in the local papers with your before and after pictures, I think you could do ok to be honest.
I would have to consider the liabilty part and look in to insurance etc but I would guess you could get around some of it with a form and the weekly diet club charge could incorporate a bike hire agreement.
The core benefit to cycling is that you can progress at your own pace and your weekly led rides would serve as an appetizer.
Of course this is probably nothing like what you are thinking of doing, but in your shoes it would be the way I would tackle it. Of course you will never make a living form one club, but if you could get it going locally seven nights a week you might have an earner