Walen attended Burlingame High School, where he began playing football until his junior year.
As a sophomore, he was lost for the season, after tearing ligaments in his left knee during the third game against the University of Nebraska.
After finishing the season with 74 tackles (third on the team) and 9 sacks (tied school record), he received Pac-10 Defensive Player of the Year, second-team All-American and All-Pac-10 honors.
On September 25, he replaced an ineffective Kevin Brooks against the Atlanta Falcons, making 4 tackles and one sack.
On October 30, he started in place of Brooks against the Phoenix Cardinals, registering 9 tackles, 2 sacks and blocked an extra point.