Originally Posted by Unca_Sam

From my perspective, you're probably really pushing for those 30 miles without realizing it, and exceeding your aerobic capacity. I love speed, and I'd push my heart rate into zone 4 regularly. The problem is that exercising at 80-90%of your max heart rate uses a different fuel than when you're exercising at 60-70%. The lower rates burn fat, which means you last longer, and since you're not creating lactate, you recover faster. My understanding is that if you train your body in the lower zones, your performance there will become more efficient, so you can go faster without exceeding that aerobic/anaerobic transition.
If you have a HR monitor, aim to just put time in at HR zone 2 a couple times a week. Don't focus on speed or distance as much as riding within that zone for an hour or half an hour. If you're riding outside or on a smart trainer, you should see improvements in your pace as your body adapts to burn fat more readily.