Go for it.
Don't worry about being left behind. It will take time, but you'll become faster as you gain experience. Next year, someone else will join the team, and that person might be afraid of being left behind. But, you, and others on the team, having been there, will be able to offer guidance. You'll make new friends.
When I was in 9th grade, a teacher talked to a kid in my class about the kid being overweight. The teacher advised him to take up running. He did. He lost weight, and became the #2 runner on the cross country team.