Definitely do it. Having an extracurricular in college makes it so much of a better experience. You will have friends and know people the day you move in. You will have upper classmen you can ask for advice on classes and professors. You won't feel pressure to get in the Greek scene just to have friends. You will have people to go to the cafeteria with and won't have to rely on the friend lottery of college dorm room mates.
Sub 20 is a mid pack time for most college girls' teams. Lots of teams would be happy to have you. Results from big conference meets or NCAA champs are not indicative of the average college female runner. And PRs on rosters are also not indicative of what the girls are running at meets. Remember that course measurement in XC can be a bit more art than science sometimes.
And the most important reason is that you will never again have anything close to the same experience available to you. The post-collegiate women's running scene in most cities is pretty terrible. This is your best chance to be in some exciting and big races.