Trying to run a sub3 marathon starting from zero

Too much reading, not enough running. You haven't been running long enough to worry about heart rate AND intervals AND 5K pace. Don't get me wrong, educating yourself is great, but right now you have too many training plans all mixed up and stirred together and not enough personal experience to test them against.

Simplify your training plan. Choose one or two kinds of workouts that you will do each week for 4-8 weeks. Then run a race and see what you learn. Then pick another race, make a new training plan, and try again. As you try different blocks of workouts, you'll get a better sense of what works for you and what areas you need to work on. As you try more races between 1500m and half marathon, you'll get a better sense of what you need to work on for a marathon. In a year or two, ask yourself what marathon training you need based on your results up to that point.

As a fellow noob with an identical goal, I've been getting a lot out of just running a lot of mileage, running 6 days a week with one tempo run, one long run, one midweek almost as long run.

Gradually ramp that 55 km/week up to 100+km/week.

Thanks! I was a bit obsessed with trying to find the "best" workouts and kept changing every week, but now maybe I'll just follow your advice and focus on one or two kinds.

sounds like you're about 3-25 years out from a sub 3:00 OP, good luck

The big goals are great, but have some closer goals and hit them, too.

Jump into some 5 and 10ks and see how they go. Then a year from now, consider running the marathon just to gain experience and get one under your belt. In that time, your goals and training will no doubt evolve a lot. Whereas just picking a big goal and waiting to shoot for it in 2+ years could limit you in a lot of ways.