It's 95% diet. However, as one poster mentioned you'll want to do longer rides. Typically for max fat burning you'll need over 90 minutes and the longer (you can handle) the better your fat burning will be. As long of rides you can handle at Z2 (HR or Power). It will be tough, but that will probably give you the most benefit.
You're way over weight however, even if other people don't think you look like you weigh 300lbs...you need to trim that weight a great amount.
Good luck.