It really doesn't matter to be honest in my opinion, set a target, if that 3.5k then so be it, just push it each time you do it, chip away at the time every time you fo it. For me its about small goals that I then work at, then set a new one. Currently I'm trying to strip time of my 5k time of 28mins which isn't fast by any standard but its my goal and its a bonus every time I take a second off it.
But high intensity will benefit any cardio workout.