I'm between 5'11'' and 6' and also started at 270. I didn't use any specific diet, since I either lack the discipline or enjoy the vice - whatever. I lost 50 lbs on a bike, but it took about 100 to 150 miles/week. No slacking on the speed, either. I joined a local bike club that and eventually found a group that was about my age and rode like a bunch of stampeding elephants - that is, loud grunting, shouting, and trumpeting. The weight began to drop fairly fast, and I got freakishly strong.

I'm 51, and it's gratifying to be able to keep up with, and quite often dominate, much younger poseurs on sub-20lb bikes, when I usually ride a 30lb roadified MTB or a 27lb tour machine, plus some baggage on the back.

Welcome, and I wish you the best results!