For what its worth, if you are actually training your mile time you want to be training a lot more than your 400m. Training your 400m will help your kick etc (and a little more than that, its not a short sprint like a 100m or 200m) but if you're training distance track you need to build up a distance base before training intervals and longer intervals like the 800m, and 1200m as well.
...that said, as someone who ran distance (cross country, winter track, track) year round for four years, don't worry about your mile time if you are training it for parkour. (If you just want to train your mile, more power to ya, but its just not something that will correlate well to improving your parkour.) If you're interested in your running ability getting better, and want to bother with timing stuff I'd focus on 400m and smaller. Running distance certainly has its benefits, but if we're talking physical jumping ability etc running the mile won't be as much help as focusing on shorter sprints.
