Sounds like that workout is actually stuck in no-mans-land in the middle where you get minimal fitness-improvement benefits. You have to both do much, much harder intervals and hit 100% max-HR, and on a separate day, go longer and do 20-minute tempo intervals. My suggestion:
1. first find your true max-HR by doing an accelerating sprint at high 95rpms+. Start at 90% sprint-effort (muscle-effort, not HR) and accelerate for about 10-sec. then increase effort to 95% for another 10-seconds, then finally, give it all out 100% sprint as hard and as long as you can until you collapse. Max-HR should be hit anywhere from 5-seconds before or after the collapse.
2. do the 2x20 LT test in the sticky above. Select a gear that gets you 90-100rpms as this will be where you'll do most your riding when at LT. This figure is actually more important to setting your training intensities than max-HR
3. anaerobic intervals are done above LT, so a HRM is not much use as your HR should be steadily climbing the entire time anyway. Intervals are typically 1-5 minutes in length at a steady muscle-intensity and speed. So a 1-minute interval could be done at 95-98% sprint-effort and a steady 32mph (or whatever you can hold for 1-minute). You should ALWAYS hit max-HR by the end of the interval. Besides the performance, strength-building benefits, it teaches you pacing and learning your body on how far it can go at various speeds. Then recovery fully to 50-60% max-HR and repeat. You'll find that as your fitness increases, the rest-time necessary will decrease tremendously. In the beginning, it may take 3-5 minutes to recover, when you're in shape, less than 1-minute.
4. aerobic tempo are another type of "interval" done at LT and below. Typically 10% below up to LT. I find these are most effective when they're at least 20-minutes in length at a steady pace, kinda like a short time-trial. Do an out & back course and get in a 2x20min tempo workout.
5. endurance is partly about energy-delivery and you'll need to do 3-4 hour rides at least once a week. Do it at a steady brisk pace that you can maintain the entire time. Typically 60-70% of MHR or around 20-10% below LT. A large portion of benefit here is learning how to pace yourself and how to maintain steady hydration and food-intake.
So as you can see, your workout is not the most effective possible. You'll want to break it up into at least three different types of workouts and do all these workouts at least once a week. The interval and tempo workouts can be done twice a week.