Well, you're obviously ignoring the fact that it doesn't have to mean he intended to catch up. The definition also clearly says "An act of pursuing someone or something." That includes any act of pursuing someone or something.
Dispatcher: Are you following him?
Zimmerman: Yeah.
So we've already ascertained that Zimmerman was actively following him (with a gun).
As for speculation, given that he was following a young man who was running (or so GZ says in the audio), and the wind noise described by the operator, it's a fairly safe guess, imo, that Zimmerman took off running to catch up. Later GZ told Hannity that Trayvon was "skipping" but not actually running, but I'm going to go with running since that's what on the tape. That's my speculation. The chasing part is a fact.