I agree with both statements above mine. To provide some food for thought here are my stats:

We started almost the same time. I bought my bike on April 1st. I'm a football player but I cycled a lot in college. It's been 20 years. I'm 42 and was around 215 when I started. I'm 185 now. I'm still 42.

I have two loops I ride often - one is a 25 mile roundtrip to the sea and it's relatively flat (Edge 500 says 243 feet of climbing - mostly underpasses). My second ride is into the mountains and it's 32 miles roundtrip and the Edge 500 says around 2000 feet of climbing.

I started out on the beach ride averaging around 16mph. Sometimes 15, sometimes 16.8, but usually around 16mph. About two months later it started creeping up. For me the lightbulb was the Edge 500 Cadence meter and Heartrate monitor. I was spinning at 65-75rpm and my heartrate was at 130bpm average. I shot for 90rpm and 150rpm on the heartrate and really upped my cardio. The average speed started increasing.

I sent from 16 to now almost always at least 19mph with a couple 21mph averages. Some days there's wind, some days there's none, some headwind both ways and some tailwind both ways. After three times a week for 8 months you start to get some ideas. Seriously I think I have 100 rides to compare data. Because it's a pretty empty bike path I can usually take traffic lights out of the equation (minus one two mile stretch with lights). My cadence is now almost always 100rpm average and my speed around 19-20mph average. I've never had lower than 17.5mph in the past couple months. So there's a quantifiable improvement there.

My mountain ride has been even more dramatic. It used to be 14mph. It's now always 17mph and ONCE 19mph. I must have been on steroids that day. The improvement is really noticeable in the hills. A lot of that has to do with the 25-30 pounds I've lost. A lot due to my improved cardio. A lot just to being more comfortable on the bike. I was on top of the world when I hit a 90rpm average cadence on the mountain (okay, hill) ride. It's usually around 95rpm now.

So I'm up around 3-4 mph on my average speed on very familiar rides in the past 8 months. I expect to climb MAYBE another 1mph by next April 1st and then taper off to nothing. Without losing another 20 pounds or doing some hardcore interval training or riding with the fast group or racing I'm going to remain stagnant.

I'm a 42 year-old ex-linebacker so maybe I'll just be happy with what I have! A new carbon R3 should be good for another 2mph, right?