You have a private coach, great. Nothing stops a college athlete from receiving private coaching on the side, so long as they pay for it. (Happens all the time in the big sports—plenty of college football QBs have "QB coaches.")
You can do both—run for your college team, and train for your Marathon.
The question is what you want to do.
Many here are right—running in college is an experience you may not want to pass up.
At the same time, it seems like you want to focus on your Marathon and do the work your private coach is prescribing.
Training would definitely differ if you tried to do both—you're not dedicating your training to just track, or just a road marathon. At the same time, though, there are plenty of runners who do workouts that cover both areas. (Heck, Emily Sisson, a favorite for tomorrow's women's Olympic Marathon Trials, missed the cut for the 2020 Olympics cut, and came back (albeit a year later) and made the 10k. Safan Hissan is another example. Heck, even Galen Rupp, love him or hate him, bounced between track and Marathon a bit in his initial transition.
So it is possible, but do you want to do it?
Also, will your coaches—because you'd have two—support this? Or would they be childish and fight over your time and attention? They'd need to be on the same page as each other, and with you, on this.
The reality is your focus would likely primarily be on your post-collegiate future, and it sounds like you want to focus on that Marathon more.
Depending on your school's program, that may still leave you as a valuable team player in track in the distance races. Heck, depending on a number of things, you still might be rocking it and competitive.
You'd also likely miss some meets, and you'd likely miss out on some of the team training (because you'd have your private coaches' prescriptions).
Anyhow, TLDR—do what you want, but you COULD do both so long as you'd be okay with it, and your coaches were as well.
Good luck at Grandma's and have fun with whatever you do, because the older you get the more you realize you should be doing stuff like this for fun. Don't lose sight of that.