My body would completely break down if I attempted to run mileage at 50-100 miles a week and instead of racing I’d be sitting idle injured! I don’t understand the insistence that marathon training needs to be on the order of 100 miles a week to be successful. Inserting cross training (cycling and swimming) is extremely beneficial as it allows extra volume without the stress on your legs.
Now the OP didn’t say if they were trying to get an Olympic trials qualifier, Boston qualifier, or how experienced they are at marathon running.
What I’ve learned, after running for now 30+ years, is that I cannot handle high run mileage, period. The MOST running mileage I did in a leadup to a running marathon (before getting as much into full distance Ironman) was 50 miles a week (5,10,5,10,20 were my runs). That was for my first marathon and I qualified for Boston, but then got injured training for Boston so couldn’t race. After getting into longer triathlons, I did my second “running” marathon and probably ran 35 miles a week at most. In that training, I swapped out the midweek and weekend 10 milers with bike rides, including a long Z2 bike ride on a Saturday. Did that one 6 years after the first, ran a little faster (but within 1 min total time).
NOW, I’m 10 years older than those two attempts and I generally don’t run much over 2.5 hours EVER while training for a full 140.6 or even Ultraman (with a double marathon the final day). Am I running as fast as I was before, not right now, and yes I acknowledge if I do want to get back to that previous running speed I have to shift the training focus a little. My shift will be towards reintroducing speedwork, track workouts, hill work, and focused tempo runs. But - I won’t be doing 50+ mile running weeks for sure.
The one thing I’ll say though to the OP- 3 runs a week won’t cut it (unless you are just trying to finish). You need the frequency of running, even if you aren’t doing high volume. I’d be looking at 5 runs a week for sure, where you do 2 mid-length ones (60 min), one long (duration depending on how injury prone you are, but my guess is 2.5hrs max), two short (~30 min). Along with this schedule, do 2-3 rides a week (2 mid week, one long). The one mid-week ride and the long ride partner with those 30 min short runs.
Yeah, I’m not saying you need to do 100mpw to run a marathon, I’m saying if you want to run your fastest possible marathon you need seriously high run mileage, and 100mpw is an arbitrary but lofty goal that *most runners can achieve after a few years. Truly the biggest determinant of marathon success is running mileage.
Like you noted, there are people who just can’t take the high mileage, and that’s fine. But I would never start out with the assumption that an athlete is capped at 35-50mpw. We would build up slowly and keep an eye on niggles and pains to make sure it’s not too much too soon, but keep the goal at 100+. You’d be amazed how easy athletes find a 50/60/70 mile week when they know the 100 mile week is coming up.
I’ve said similar things in the past about training volume, mainly that for amateur athletes the limiter is almost always available time, not fatigue. In this case, I would argue the best thing you, OP, or any triathlete trying to PR an open marathon could do is spend a month or two shifting as much work into running as possible. If you can to 100% running that would be awesome, but I realize running volume is pretty much capped at ~12hr/wk for most people. I would then question the rest of the sessions and if they are adding to or detracting from running fitness. Hard cycling workouts are fun but will ultimately detract from your ability to do the important run workout.