It's mostly just the contrast between how I learned running in High school cross country and what's actually recommended now. There were no real rest days, we ran 5 days a week and we were supposed to run at least once on the weekends. We ran hill reps two days a week, and long runs on the other days. We were all on the same training program regardless of where we started from.

What I've read recently is that about 4 days a week is a better way to do it, at least during your early progress, with a mixture of long slow runs and some interval work outs once you've reached a good level of fitness.