Marathons and all the other road races will always be there. High school track may be your only opportunity to be apart of a track team. You'll remember your team for the rest of your life. If you go the marathon route, you'll train, run the race, experience a bit of first marathon pain, and move on.
If you're not fast, be the teammate who shows up and works the hardest, cheers, and supports everyone. Watch the movie McFarland USA. You could be the Danny Diaz of your team. You'll see how important he became to the team. Is it cool to be Danny Diaz, no, but without him, the team wouldn't the same. Teams are great, but they need everyone to do their part. Not everyone is breaking 4 minutes for the mile and winning race, but the team needs everyone to do their best trying to score points.
I recommend you to do these:
1. Show up every day, even early and stay late
2. Train hard
3. Race your best
5. Do all the little things (stretch, foam roll, easy runs, rack the jump pits, pick up hurdles and other field stuff)
6. Give up high fives and support your teammates
7. Learn about running theories (ask the coach questions)
8. Put in the long easy runs and the slow recovery runs with your teammates
If you do these, you'll get faster and may even improve more than other teammates. You'll get more out of track and cross country for your life than training for a marathon by yourself. I'm sure that you'll learn and become a better teammate, person, and a life time runner. I'm corny and I love the marathon, but track and field and cross country teams is more training for life.