The oldest and cheapest way to get faster is to ride with faster riders. If time constraints prevent that (and I remember what it was like to find time to ride with babies in the house), your best bet is interval training. Most of the programs are set up to use with powermeters and heartrate monitors, but you can still do them with just a timer function on your bike computer.

The tried and true interval for raising your FTP (roughly the highest average power you can produce for 60 min) is 2x20' with a 5' rest between intervals. This is just spitballing, but I think it may be doable for your situation. You say you can do the 30mi. loop with a 17+mph avg. speed. For your 20' interval try to maintain a steady 20mph. It should feel heavy, but not killing. I think of a 7.5-8 out of 10 on the effort scale. Then 5 minutes of coasting/soft pedaling and do it again. You can do these several times a week and continue to do your long rides.