I've done ok on low mileage, but how much more do I need to be respectable?

You’re focusing too much on the coach and the cost. Build your base first and than worry about that later

So you are in your late 30's right? I really respectable time in the marathon for someone at or approaching 40 is 3:00. I was a 2:33 runner back in the day and I set a goal to run 3:00 at the age of 40. I fell short and ended up with 3:07 but was happy with the result. I don't know what natural ability you have or what limitations you might face in terms of durability or injury vulnerability but if you are doing the "right" workouts you should be able to come very close to your potential at between 50 and 60 miles/week. Before simply ramping up your mileage much further than this it would be good to have an extended period of injury free increase in volume. The 3 workouts I have found that will maximize marathon potential are (a) a long run - I no longer feel compelled to go as far as 20 miles but gradually building up to at least 16 and having alternating weeks of something shorter like 10 or 12 (b) long intervals or hill repeats - running for 3 to 5 minutes at 90% of your maximum heart rate x between 4 and 6 intervals (c) a "tempo" run which would gradually increase from maybe 4 to 8 miles at about your half marathon race pace. If you can fit these 3 workouts into every week and then run between 4 and 6 miles on your off days but at a pace that allows you to recover you should get to very close to your maximum potential. Simply increasing mileage with slow running to 70 or 80 or more without covering the bases and working to increase Max VO2 and efficiency at speed is not the best use of your time. I ran 2:33 off of 50 miles/week - would I have run faster with more mileage, certainly - but at the time I was a West Point cadet busy marching and with a crushing academic workload

I know. There are men/women running 2:45-2:50 that are in their 50s. I have no excuse not to be fast.

I'm going to be 38 in November.

I'm also very hard on myself.

I was not fast in HS at all but considering that as late as 8th grade (age 14) I did NO exercise whatsoever!

I'm throwing in more tempo runs/fartleks too, and really slowing down my recovery runs. Like I'll run 7 miles, and miles 3-4 are at 10k pace or so, but miles 1-2 are EZ/warmup. Miles 5-7 are moderate/cooldown, for instance.

Or I've been doing it like miles 1-2 EZ, mile 3 hard, mile 4 EZ, mile 5 hard, mile 6-7 cooldown, etc.

I haven't yet worked in track stuff because I want to see how effectively I can hold 41-43 (30 mpw Sun-Fri, 10-12 on Sat.) with no injuries.

I mean, hell, if I ran 1:39 off 25 mpw in 2018 for a half and the longest week I had topped out at 38 miles, with only 3 10-milers and 1 11-miler, I want to see what I can do with a proper base build to 50-60 over time, and gradually.

I never ran past HS. I know most here have D1-D3 experience too, so I can't compare myself to others as much.