45 mpw isn't enough for a marathon? Depends on the individual, and what they want to achieve. Didn't do me any harm and achieved decent results. The OP has a speed bias, and no endurance and/or pacing as you say TR .
I agree its a lot about how much you want to commit, but so many plans are pace based, and some people need the affirmation that they are on schedule and target - that is how I operate. If I am using an 'off the shelf' plan then I will set a target, and adjust periodically. Part of the skill is nowing when to drop sessions, back off the pace etc - this bit is all new to me!
For me, week 0 hadn't run for 25 years, week 21 ran 3:02. Didn't faff about with much speed work. Just focused on endurance - weekend long runs with sections at your marathon pace, and easy runs and the odd tempo run midweek. Averaged 40-45mpw.
Went on to do approx 60mpw for 16 weeks, more in line with P&D. It was a lot of extra running for small gains in my marathon time. I got more satisfaction from the 45mpw campaign as I was fresher, and less prone to injury, more than I did with the sub 3 campaign. Others need the mileage - Nell McAndrew's sub3 being a good example - she really stepped up the mileage last year.