Flagpole Admirer
As Gibbons says: run.
I walked on at CU. The coach didn't know I existed either. I was the only guy he let walk on that year (he told a guy that had run 4:40 and had been running 70 mpw all summer to go run 90 mpw for a year and see him the following fall) and it was a great experience. One of my teammates had walked on two years earlier - the coach made him give his stuff back after his first year! - and as a senior he scored on the only CU men's team ever to win heps.
You only have this opportunity once. Do it. You will be glad you did.