Depends on your commute, but your distance matches mine (though mine is 6 days a week). If I set it up right, I could do much of my high-intensity training during my commute, or by just extending it slightly. I could easily do something like Carmichael's low-volume plan (check out the Time Crunched Cyclist if you're interested). I typically have some goal in mind for my commute. On the way in, it's 5 am and traffic is very light, and the wind is mostly consistent from day to day, so I'll try to beat my best time from point a to b or something. On the way home, if I'm not tired, I might do sprints between stops.

Longer rides are best for...building the endurance to do longer rides efficiently. I usually include some kind of intervals during them if they are not group rides (which kind of have their own intervals built-in). I treat my long rides more as enjoyment than work, though. And when you get better, you really notice it from about the 90 min mark on.

If you want to push it on your loop, try negative splitting it. Do the first two at your normal speed, and try to best it on the third. Or do that on the 2nd when you're more fresh. I often try to negative split my rides. Keeps me from just slogging home, and lets me compete against myself.

BTW, 20 minutes over 50 miles is a real improvement. Acknowledge your own accomplishments. Congratulations.