Stuck at 18 mph...

That's where I am, stuck at around 18 mph average speed when cycling along the coast.  I'm only 35, it's mostly flat, I'm not that bad, but I can't seem to get to that 20 mph level.  All I want to do is blame stoplights, stop signs, weird traffic patterns that happen when people going to the beach try to parallel park, etc.  But that's not the reason.  I'm just not sure if it's the legs or the lungs.  The big is fine, a Cannondale EVO isn't exactly the slowest or heaviest bike on the planet.  I've been looking around other blogs and evidently I have to learn to ride in the drops, do interval training, etc.  Maybe the"rest days" are going to have to become "stretching days".