Agree with the consensus posted in this thread. If you want to run a marathon, let alone race a marathon shooting for sub-3, you need to be running more than 3 days a week, full stop. Could it work? Sure, if you have the natural ability, training history etc. But is it the “best” approach? No way in hell. The reason that plan calls for running 50% of the time at pace is precisely because it’s only 3 days a week. But as any coach / plan will tell you, you can’t just substitute intensity for volume, especially when the event itself is extremely long (aka “specificity”). You need to run enough so that running becomes second-nature to your body. Easy runs should feel, well, very easy by the time you’re halfway through the build. This takes consistency and volume. If you run every day, even if it’s easy and short, your body will adapt. This will enable you to hit your “key” sessions without incurring a huge recovery penalty.
I just finished my first marathon build off of a season of 70.3s where I was averaging 20 - 30 miles per week for most of the year. Ramped it up to 40, 50, 55 the peak week, and that was with cutting out my swim and very little biking (I would normally advise to keep some biking volume, but I was trying to avoid burnout at the end of the year). For reference I ran a 5K PR the previous year of 17:55 and ran a 1:29 on a flat course in my last 70.3. I ran 3:01:30 on a hilly course for my marathon (BQ for me). It’s a different animal than a HM or 70.3.
If you’re a triathlete, you can definitely supplement some running with cycling. Cycling is excellent for general aerobic fitness because you can go long and slow with zero impact. Running 2 hours is extremely taxing even if going slow, a 2 hour Z2 ride on the trainer is easy peasy. I would also look into low-cadence work, and finally strength training, especially if you’re older, can really make a difference. I would check out plans from McMillan, I’ve had good success with them and they’re well-known and respected (nothing crazy). Good luck!
Thanks a lot for your comments. What I can draw from this discussion is the fact that a really fast 5k does not necessarily translate to a sub 3 marathon - well in your case probably the 3.01 would be sub 3 on a flat course. I have looked into the McMilan plans they are focused on volume - not sure I can work this much running into my schedule. With all the collective feedback I have got I will likely try to go for a “hybrid” and have 2 longer runs with a bit less intensity and 2 race pace focused runs per week. I don’t see myself running more than 4 times per week (due to my work schedule). I’ll see how it goes…