1. Do not underestimate that glute/butt impacting everything else with your running. I've been battling glute issues for a while now and it is far and away the worst and slowest healing injury. I wish it was an easy way out of this and I'm getting worried this will be impacting my running forever.
Here's more information about the importance of the glutes with running.
2. I think leg strengthening would be good, but I'm seeing that sometimes they inflame my angry glute. I'm trying to carefully navigate it and make it a bit stronger. Paging
@MAC_32 if he has a few good options to carefully strengthen my glute. And if it would be better to carefully define it by doing lots of reps of easier stuff as opposed to 6-10 reps of something heavier.
3. I strongly recommend you incorporate core workouts (crunches, etc.) every other day into your plan. It only takes 5-10 minutes of a good focused workout and it will help your running and hopefully help your glute.
4. I'm a Hanson guy and if you followed that program, I don't think you need more tempo runs to improve your marathon. I don't think that is your problem.
5. Before COVID, I ran a 3:12 marathon and I could barely break a 20 minute 5K and I could run a 1:31 HM (and probably closer to 1:29....but I never did, so I'm a 1:31 HM guy until I prove otherwise.) I'm a slow twitch muscle guy, so I excel better at longer races and my shorter race times are abnormally slow compared to my longer race times. You MAY be the opposite. But your 1:39 HM suggests to me you are faster than 3:56 marathoner. Some say double your half marathon time and add 10 minutes. In most of my cases, I've needed 15 minutes. Regardless, with that math, it brings you in way faster than 3:56.
6. For me, my PR marathon, I finally feel like I started to figure out nutrition. Both the week of the marathon but the 24 hours prior to the race and the race itself. If you don't do your nutrition right, you will have a rough go of 26.2. Your body can mask bad nutrition over 13.1, but it gets exposed big time at 26.2. The day before the race, I ate lots of pasta at lunch and then went with a light salmon and rice meal for dinner. I drank pickle juice the evening before and the morning of. I continued with a bagel and peanut butter the morning of the race. For my marathon, I religiously took in calories every 4-5 miles. I had always waited until mile 6-9 in my marathons, and did it when I felt like it. I was stronger (I think) because I was methodically putting calories into my body (Maurten Gels, ShotBloks, and bananas) during the race.
In summary, if you can heal your butt and you work on your nutrition, I have to believe you will shave 10-15 minutes off of your marathon time in the blink of an eye. But do not despair, marathons are challenging physically and mentally. These challenges that we face are what make them so rewarding when we figure out what works for each of us. I'm confident you will figure it out. You have too much talent and drive not to. Can't wait to watch you crush it in the future.
Hope this helps a little bit.