Do you ride with anyone? It's relatively easy to keep up with others who are faster than you on the flats (versus the climbs, because of the draft and relative difficulty). Riding on flat rides with a few friends to a big group that is slightly out of your comfort zone and that have some good surges thrown in will force you to push yourself more than you would solo. That is provided you pick people who can be your rabbit and not just leave you in the dust. That's a great way to improve speed. Don't do it constantly, because it can get demoralizing to always be chasing. Just work it into your training routine if you can. It took me a long time, but I started doing those kinds of rides last summer back in my hometown, and then I had to keep up with the guys on training rides through he fall, and then when I went back home this spring, they said I had improved 100%. It does help to have some nice people willing to give you a push now and then when you're suffering, like they do.