He was XC Champion well after school. I assume when talking 'walk-on' by default we are talking NCAA.
School gives no athletic money:
Recruited: Not a walkon
Not recruited: Walkon
School gives athletic money:
Has scholarship: Scholarship althlete
No scholarship: Walkon
However there is a blurring between the two when an athlete has been recruited but was not given a scholarship. For all formal and legal purposes, they're walkons. Although they don't exactly fit the normal definition since they were recruited. So I often here of such cases described as "preferred walkons" or "recruited walkons" although I don't beleive that there is an official term for that.
I was a walk-on at one of the Ivies in the 1980s. At the time, we referred to "walk-ons" as the kids who hadn't been recruited. "Recruits" were the kids who the coaches had contacted during high school, advocated for during the admissions process and urged to attend the university.
Basically, recruits were the kids good enough so the coaches did the selling job with the admissions office to try to get them admitted (not always successful...I saw some promising runners who the coaches really wanted but the admissions office said no way) plus the selling job with the athlete to convince them to attend if they got in.
It was basically a matter of who got attention during the admission process. Financial aid was separate. You could have a recruit who got no tuition aid and a walk on who was on full aid.
That's how I remember it. Bear in mind that I wasn't recruited and I'm going on stuff I picked up half a lifetime ago. Also this was 20 years after Shorter's time.