If you want to improve your ability to ride at high speed on flat roads, your best bet is to do just that: ride in a fast group on flat roads. This is great, because riding with a group is fun and good for fitness and skill development.
There are also some specific training drills that you can do to increase your speed.
Matt Stevens came up with this 1 minute training exercise:
Essentially, the 1 minute interval is a 1 minute on, 5 minute off sprinting interval.
Find a small hill (or bridge if you live in Florida) and get up to around 30mph while descending.
Once you hit the bottom of the hill keep the 30mph (50kph) speed up for one minute.
Recover with a very slow and steady pace for 5 minutes. Repeat this interval between 8 - 10 times.