Congratulations on getting back into cycling!
You're putting in a lot of distance each week; are you riding at a steady pace most the time, or are you doing different sessions on different days, varying the intensity and/or speed?
You say that you've made improvements, (with the distance you're doing I'd be very surprised if you hadn't) but how are you judging these improvements? In addition to an overall improvement in your fitness level, have you achieved body fat reduction? Are you assessing the improvements on a time/distance/effort basis?
Is there a local cycling club that you could join, or do you have a local group ride? If you do one or both of these things you can assess your fitness against that of other riders, and maybe pick up some training pointers while you ride.
What I'd suggest that you consider doing is to contact a licensed cycling coach and having them conduct an assessment of your present state. Once you've done that you should have the coach develop a training program that is customized to your needs. The relatively modest costs involved will be repaid many times by the optimization of your training.
Good luck!
