not to be discouraging and trying to be humorous but flatness is relative and nowhere is really flat. Even if it is totally flat it can feel uphill or at least a false flat where you have to keep down shifting because of wind, fatigue. etc.
Keep riding the hills, they make the flats more truly flat.
congrats on the downhill speed, I hit my best ever speed last summer on the Mississippi ride- a whole 36 mph. I was afraid to let myself go faster. I kept envisioning a rock in my path and what would happen. Basically a coward I gues. On the other hand it did go on for about 4 miles so at least it was long enough to remember.