You don't build speed over night. It's a process. Your diet and recovery are also important so your body can build muscle properly. Training is reasonably complex so read up on all aspects of training. Realize that if you go from being able to ride at 15mph to 16mph that is a 6% (roughly) improvement. That's a pretty good improvement in itself and even that small improvement may take time and still be subtle enough to hardly know it has happened. Doing the kind of interval training that chasm54 and others have outlined here is not fun. It is seriously difficult and exhausting so it has to be interspersed with actually having fun or you'll burn out. Old geezers like myself don't have a lot of testosterone coursing through our bodies so building muscle is double slow. I'll never be objectively fast....but I train as hard as I can and still enjoy the experience simply because I like challenging myself. Best of luck on your quest.