Too many hills to give an accurate number. I use to ride a dead flat perfectly paved rail trail regularly but haven't been to that trail in the past year and a half. It's a perfect trail to determine what your cruise speed is.
Started out on an old rusted creaky Walmart mountain bike 3.5 years ago on that rail trail. I cruised at around 13 mph on that POS. Bought my Giant hybrid and increased the cruise speed to around 15-16. Through that year I probably increased another mph to 16-17 cruise speed.
Been riding for the last year and a half mostly road with a ton of hills and a lot of non-flat longer distances trails thrown in on weekends. Nothing flat enough to give me an idea how I would compare now to the first full year of riding.