It is a good jump to go from 15 to 18! It does take time, yes, but you can do it. You said you ride mid-pack? Can you ride mid-pack with a faster group, and stay there for a significant distance? If you're comfortable doing that, getting on the wheel in front of you, and I mean right on the wheel, and staying there, is a "cheap" way to go faster - let them pull you. However, to do it on your own is going to require more fitness. Ride more often and for longer periods of time (not worrying so much about mileage), do intervals (look that up), and climb more. Sprint away from stops and try to maintain whatever speed you get to for a hundred yards or so. Warm up before you ride. Hydrate before and while you ride! And get rabid (that's the fun part)!