50 MPW for slower runners and % weekly mileage

I would max out a few weeks before the marathon next year but not be higher than 70 by then, but would get up to at least 60-65.

A typical week could look like

Monday 7 miles easy

Tuesday 4 miles of warmup/cooldown and 3-4 miles of 5k/10k or hill reps

Wednesday 9 miles easy

Thursday 7 miles easy

Friday 4 miles of warmup/cooldown and 5 miles worth of threshold, either several few minute long reps, or 1 mile reps, or 2 mile reps or maybe a 5-6 mile progression or 4 mile even pace threshold

Saturday 6 miles easy

Sunday long run of 12-17 miles (don't always go the same distance and sometimes run a good chunk about 20-30 seconds slower than marathon pace)

That's 60ish miles I believe. Don't do the same workouts every week, but keep them in those type of styles, a shorter faster one, and one that's more threshold/half marathon paced. You can add in some strides on one or two of those easy run days. If you want a rest day, you could take off one of the easy run days and add a mile to the other days.

You're buying the furniture before you've built the house. Just worry about getting to 50 a week. When I was in the 50-60 range I typically did a 6-8 mile run five days a week and a run of about 15 miles once a week. On the seventh day I did whatever I wanted, I sort of recall something like four miles being pretty common. That's all I did for about three months though a lot of the runs were hilly. Then it was cross country season and there were a lot of intervals and races, probably too many intervals but that was the era.
It's easy to get caught up in all the physiology stuff and lots of people love it. But running is a very simple sport. You can do very well keeping it simple and I think everyone is better off doing so at least until they've been around a few years and have spent time making heads and tails of it. Nic Bideau has coached many world class distance people and is the source of one of my favorite quotes; "If you just run for an hour a day you'll get very fit. If you extend one of those runs to 90 or 120 minutes you'll get even fitter and if you extend a second of those runs to about 90 minutes you'll get very, very, fit." Work towards that, maybe progressing from the first suggestion to the second and eventually the third as you get fitter. Adjust it as you need to, push yourself but don't kill yourself, if you feel like running faster now and again do it but don't complicate it. Good luck

I like to build mileage by doing things by time. I've had some success with starting at 4 hours/week and build up adding 15-30 minutes every week. Chilling at 6 hours a week now for a bit. See how this works for ya

A couple of years ago at age 65, I ran 19:46 on about 28-30 miles a week.

I was doing three or four runs a week of Easy Interval Sessions (3x2000, 6x1000m, 12x400m and 16x200m) with a couple of 3 or 4 miles easy runs (about 8-8:15 per mile).

So, we're talking about 80% of weekly mileage at half marathon pace or faster - the 1000m at 10k pace; the 400m at 5000m pace; and the 200m somewhere between 800m and 1500m pace.

It would be 100% of training at quicker than marathon.

I will say I'd a 50 year background to draw on, which might make a difference.