Although I do agree that pre-interruptus he definitely seemed to be taking the lead and Jack was more passive, Ianto's run after Gwen, in my eyes, appears weaker than Jack's saunter out after him. He seems flustered and a little embarrassed to be caught, though he hides it well. I think, like halfspokenwords said, Gwen's appearance suddenly changed the dynamic to "work," which threw him, not to mention he's a very private person who's just been seen in a very personal and exposed situation.

I think he was running out 1) to possibly do a little intercession/damage control, 2) see what she wanted, and 3) tell her about the package. All of which is very much a way to reestablish his work persona and hopefully distract Gwen from what she's just seen. That's how that run read to me.