I don't disagree with much of the advice you've been given.
For a first-time marathoner, I think it's important to have at least one run that has a duration about as long as your target time -- close to it, anyway, without worrying overmuch about speed. If your target is 4 hours, get out there at a comfortable pace for 4 hours at least once. No need to hit it hard. The reason is that you're going to find some mental and physical territory that's new to you when you've been moving for that long. If you get a good sense of what that's like, then in the race you'll have an idea of what to expect.
When I lined up for my first one (3:49) in my late thirties, I had no idea what lay out there 20 miles and beyond. I'd heard about The Wall but hadn't experienced it. Then I learned what those last 5-6 miles could be like, and how to handle them, and fairly quickly got under 3 and then to 2:51 (but never below, though I tried).
Another thing a long run helps with is equipment and your body parts. For example, does that singlet you're going to wear cause chafing after 4 hours? Shorts? What happens inside your shoes when you've been running that long? Those are areas where you don't want any bad surprises.
I'm about to do (yes, "do," which means mostly walking) a marathon 40 years after my first one, and about 10 years after my most recent official one (I've gone over marathon distance many times by myself). Here's the thing: when I go to the line, I know that if I don't do something stupid, there will be no surprises. I know what my body does at that distance and over my target time (which is pathetically slow, but hey -- at least I'm still at it, and in a race I go as fast as I can).