I started back to running 2 miles about 3 time a week about a year ago. In the summer I ran 35 to 45 mpw. Most runs were 5 miles with some several 12+ mile runs on weekends. At 51 I BQ'ed at my first marathon in Sept with a time of 3:24 and went for a 2018 BQ in November and got a time of 3:10. I am 6'0" and weigh 165. My speed work was limited to running 5 miles with a local running club at 7:02 to 7:10 pace once every week or so. I followed no marathon training plan. I only attempted a BQ after running a 10 race at a pace that indicated I had a shot at a BQ. I ran my longest run of about 16 miles on a group run 6 days before the first marathon to see if I could run 16 and keep a on pace that would give me a shot at a BQ.
Now that I am training for Boston I have upped my mileage to 70 mpw on two different weeks and hit 80 last week. I don't have the time to do much over 50 mpw every week and my body thus far cannot maintain the higher mileage without pain coming back to areas where I have injuries in the past so I back down the mileage for a week or more after a high mileage week.
So I don't think everyone needs sustained high mileage or needs to follow a marathon training plan to BQ. It probably helps but I somehow did it with less. Just may sure you pick a marathon that is flat and where you have a shot at decent weather. My first marathon was on a warmer and more humid than normal day for that time of year. Only about 30% BQ'ed rather than the prior year's 60%. My 2nd marathon started in the upper 30s and ended in the lower 60s.
I ran a 5K in the spring in the 20s in the spring and 19:15 in the fall.