Originally Posted by
Jimbo47
Riding a cruiser you have different muscles that come into play. Your back pain is more than likely because your abdominal and core muscles are carrying more of the load than stretched out over a road bike frame.
Try to shorten your trips at first, and gradually increase as to help build up strength in those muscles.
Exercises to build core strength will help as well.
This and weight loss. But no one size fits all. Position yourself so it doesn't hurt.