Raffy,
I think you'll find that the more you ride the more you realize how hard you can push yourself. For me, the first year of riding was really slow. I bought the Roubiax in the spring of 2011 and I looked back through that year and there wasn't a single ride where I averaged much over 15mph and most of my rides were 10 to 15 miles. Heck, even through the first half of 2012 I rarely went much over 16mph. It was like all of a sudden middle of the way through last year a switch got flipped and I added 2mph to my average. My best guess is that it was a direct result of riding 60 to 80 miles a week, generally about 20mi at a time (when I started commuting to work).
But I also don't always ride that hard. I still go out and try to do tempo rides in the 16's because I know I let myself get so badly out of cardiovascular shape that I need to log miles at a lower HR for my own good. So really my goal for this year is not to add any more speed but endurance. If I can keep the current frequency and duration of rides up for the next month I think August could be pretty interesting from a testing my limits perspective. And this fall for the 3-notch ride there's a group of guys coming, one of which is a good friend of mine, who are all triathlete nut jobs who can really hammer. I'd like to think that I might be able to hang with them. We'll see. Looking back at the last three days of riding it's pretty nutty that I did 95 miles at average of just shy of 19mph. Time to slow down for a week or more.