Keep riding the Trek. If you know for sure that you are going to get serious with the hobby, then test ride carbon, aluminum, and steel bikes to zero-in on your preference. I favor the older 80s Cr-Mo road bikes with high-end tubings. They are very durable, reasonably light (21-23 lbs), and have good damping. Older 105/600/Dura Ace side-pulls have plenty of power to flip the rider. You can buy an excellent used bike on CL for about $500. Spend another $500 if needed to upgrade the drive-train to match your road terrain and strength level.

I upgrade the drive-train to 8-speed cassette and rear trigger shifters for greater durability and flexibility to match my ENGINE. You can slowly upgrade the Cr-Mo bike as you see fit. There will not be a significant decrease in commute time with an 18 lbs road bike. A drop of 4 lbs is less than 2% of the average weight of rider and bike. You may gain a few seconds up a hill, but will recover about 1/2 of that time as potential energy is converted into kinetic energy on the way down. Keep working on the ENGINE and climbing technique instead of focusing on an ultra-light bike.

Adjust the bike so that you can ride comfortably and work efficiently to put power into the ground.