Not really any advice on what to do but
When I was 39 I ran in the high 16s low 17s with pretty much the same training you have been doing. I got down to 75s for my 400s and 2:40 or better for my 800s and did 5 mile tempo runs at around 6 minute pace. Usually ran 3 days a week for 28-30 miles and did the stairmaster the other days.
After a long non running injury, I came back at 45 and ran a 23:20 4 mile which age grades about the same. I did absolutely no speed work or tempos. I would run a hard hilly hour on the roads, a hard flat hour on the roads the following day and an easy hilly trail run the 3rd day. I would take the 4th day off and then repeat. I then did a lot of races and watched my time keep dropping.
A couple years ago I got in pretty good shape doing two runs every week averaging 15 miles per run. My 5k times were slower, but I could run great races from 10-15 miles.
At 49, I have been averaging 65-70 miles a week but not racing any to see how much I can drop my times when I turn 50. I would be thrilled to get under 18 for 5k.
The only thing you can do is gradually increase either the volume or intensity of your training and hope you stay healthy and keep improving. It gets much harder the older you get.