Question: In one of the cut-away scenes, Peter is riding a bike. He tells Brian that he is "one of those people now." What does he mean by "those people"?
Answer: This occurs in a flashback where Peter is recalling a time Brian discovered his "hidden shame." In the memory, Brian is driving a car. Peter pulls up next to him on a bicycle. He is decked out in a full, multi-neon-colored lycra spandex pro bicycling get-up, with matching reflecting helmet and some kind of rear view mirror attached. The ensemble is complete with riding gloves and the latest athletic shoes. When Brian notices it's him, he exclaims, "PETER?!" To which Peter responds, "Brian, I'm sorry. I'm one of these guys now...I'm SORRY, Brian. I'M SORRY! (as Brian drives off, aghast) " By "these guys," Peter means the kind of guy who, though a casual cyclist and nowhere near professional level, still buys all of the latest riding clothes and gear, making him look silly and pretentious (in "Family Guy" terms, he looks like a "douchebag").
Answer: I took it as an annoying bike-guy in general. Since professional or not, they're all annoying.