Lots of great stuff here. I'll break up your text into more digestible points.
Realistically I could run 6 days a week if I could keep it 60-90 minutes. Would that be more beneficial to the marathon training, adding additional days later in the training, since I can't add mileage daily?
I think this is the most important part of everything you said. If you're willing to run 6 days a week for between 60-90 minutes, then I would do this. You will likely see the best benefits from training using this method. But we'll come back to this as running 6 days per week at 60-90 minutes is where you'd want to end up and not where you'd want to start.
I can't seem to do better than a 15:00 min/mile where outside I am finally around or under 12:00 min/mile. Are you saying that I can continue to do 15:00 min/mile 3 days a week at 60-90 minutes each, then up my long run day mileage by no more than 35%?
Warning math is coming....
A way to look at it:
You can run 12:00 min/mile comfortably outside. We'd base you paces around something like this based on current fitness level and recent race times. But as an
example only, let's say 12:00 min/mile ends up being your long run pace. If you max at 150 minutes (assuming running only and not run/walk) for a long run, then a 12:00 min/mile maxes at 12.5 miles (150 min / 12). To be 35%, this would mean you'd have a minimum weekly mileage of 35.7 (12.5 miles / 0.35). Since you're running 12.5 as the long run this leaves you with 23.2 remaining miles on Saturday and 4 weekdays. The 4 weekdays are 15:00 min/mile at 60 minutes (or 4 miles). That's 16 miles. So 23.2-16 = 7.2 miles for Saturday minimum (which at a 13:00 min/mile pace is roughly 94 minutes).
How would I increase my mileage as I get closer to my marathon? I can get in 3-4 weekday runs if I run 60-90 minutes. But Higdon's plan calls for increasing the mileage during the week up to 10 miles and I just don't have 2-2 1/2 hours a night to run on the treadmill.
Most generic running training plans are written for the general masses. They're made to be easy to look at and easy for people to figure out without much outside input. But it doesn't mean that the timing of such plans works for everyone. Ideally everyone would write their plans in terms of total time used to complete runs and then the relative pace at which they should run it. Like this:
Monday - 45 minutes @ 60% VO2max
Tuesday - 60 minutes @ 75% VO2max
Wednesday - 15 minutes @ 50% VO2max + 25 minutes @ 80% VO2max + 15 minutes @ 50% VO2max
The problem is this looks very confusing. And when I first picked up the Jack Daniels running book he puts things in terms like this, but boy did it look like another language to me. I ran scared from that plan. But his methodology makes more sense to me now. It's based on time and relative pace based on your fitness.
So when Higdon says 10 miles. It's more like he's shooting for the middle of where he believes people that are choosing that plan will be in terms of time needed to complete 10 miles. Thus, a generic plan is usually not the best plan for everyone. I would completely agree that a 2.5 hour training run mid-week is brutal and very few people could realistically fit that into their schedule.
So, as I tell everyone, I'm willing to help you set something up that's custom to your schedule, desires, and current abilities that I believe will give you the best chance at an enjoyable marathon experience. Just send me a PM if you're interested. I do it free of charge because I want everyone to have the best running experience possible and find it to be a fun hobby trying to get all of the pieces to fit together.