I was you last year. I first started last year in August with a Walmart mountain bike. I swear it is 20 lb heavier than the Giant Escape 1 I bought this spring.
I started out riding too high of a gear and sustaining 10-11 mph for an hour. How are you pedaling? Are you pedaling at a fast rpm or pedaling slow in a high gear (this is your cadence, the rpm you pedal.) I read about cadence here on the forum and even posted a question about it (
here it is if you want to look, I got a lot of useful responses that helped me understand it all.)
I was pedaling in my top gear on my junk mountain bike which for the speed I was running was about 50 rpm. After reading the responses to my post, I started using lower gears and pedaling faster. It felt wrong at first, but as I got use to pedaling at a faster cadence, I started riding at a faster speed. I finished last year when the weather turned riding about 80-90 rpm and riding that heavy bike about 16-17 mph. I also was riding about 3 gears lower than when I started. This was all on a perfectly flat bike trail.
Now that I have my Giant Escape, I have only been on that trail twice and it was with my daughter, so I don't know what my speed on the new much lighter bike is in comparison as I am now riding rollercoaster hills on the roads around home. I do know that I topped out that old mountain bike spinning the pedals like mad on a downhill to about 22 mph in top gear before I couldn't spin them fast enough to keep up while my Escape I hit 33 mph on that same downhill pedaling and still have 2 more gears to go. On the Escape, I usually leave it in the middle chain ring on the front which can get me up to 28 mph easy on the highest gear in the back going on a bit of a downhill.
I still remember month 1 of my starting on the bike last year. I rode almost daily and it took 3 months to progress from cruising at 10 mph to cruising at 16-17 mph (on flat.) Note, I am considerably heavy so 220-225 lb doesn't help the speed very well. Unfortunately I am now only averaging 9 mph with the new bike because I am on rollercoaster hills where every quarter mile I am constantly switching from 28-30 mph downhill for 10 seconds to climbing at 6-7 mph for 5-10 minutes. Those same 6-7 mph uphill quarter to half mile climbs that I ride now, last year were 2-3 mph climbs with stopping for a break on the mountain bike, so it's quite an improvement.