Mike - First, welcome!

Bad news up front: You know those guys who effortlessly fly by you? If you switched bikes with them, they'd still effortlessly fly by you. They're fit, and it was a lot of hard work, intelligently directed, that got them there. You won't match them with the time you're putting in. If you're averaging 18 mph on your rides now, though, you're doing quite well.

As for equipment, tires have already been mentioned. Another relatively inexpensive change you can make it the cassette. If the poster above is correct and your current one is a 11-30, you can have your bike shop switch it out for a 12-25. It won't make you faster, but it will reduce the gap between gears and make it easier to maintain your pace over undulating terrain. That cassette range (12-25) is common on ten speed cassettes as well - you just have fewer gaps with the extra cogs.

I just retired from the Army and Army National Guard after almost 37 years of service. My retirement present to me was a high-end bike worth almost $5k (I paid much less - found some great deals). That was in July. A couple of weeks ago, I bought a bike off of Craig's List for $300. It's an eight speed Bianchi steel frame bike made in '97 that needs a bit of TLC to get it back on the road. It weighs a lot more than my main bike. The parts were good quality then, about the relative level of current 105, but not top of the line. The expected effect on my average speed? Less than one mph, if that.