Hi gbabb!
Welcome to RPM! While we ride the beat in tracks 3, 5, and 7, tracks 4 and 6 are more individual. That means you ride at YOUR top pace. You're on the right track by adjusting the resistance and slowing down your pace. You'll need to experiment a little to find what's the right resistance and pace for you at this time. Some things to think about the next time you're in class...
1. Control. Our mantra, "Never sacrifice control for speed." Stay in control of the bike. If you're bouncing around in the saddle, you are not in control. You are in control when your upper body is relaxed but stable and your hips are firm in the saddle. We stay in control with proper resistance, strong core muscles, and great technique.
2. Perceived exertion. You should never be maxed out at the end of a track 4 or 6. Theses are primarily recovery tracks to get yourself ready for the intervals and mountains. Uncomfortable or moderate open mouth breathing is where you wanna be at the end of track 4 and 6.
3. Pace. As your fitness increases, so will your top pace. As your technique improves you will ride more efficiently, which will increase your top pace. As your leg muscles get use to the higher cadences, guess what? Your top pace will increase. Basically just be patient and keep on riding!!!! 
Hope this helped. Keep us updated on you progress!!!
-Mark