I've had issues with my running form before and have had 4 different coaches work with me to change it and although i tried i was unable to do so. I have the opposite problem i'm a toe runner and they said the same things...that i'm wasting too much energy and its bad for my legs and so forth. I've managed to stay pretty injury free and have been running since the 6th grade. Sometimes your running mechanics are biological...i run the same way my dad did because of the way my knees are. Didn't stop me from getting a running scholarship LOL . So if you try to change it and you can't but your still running fast and are comfortable dont worry about. Everyone runs a little differently!! Whenever i'd finish a race and someone would tell me i ran funny i'd just remind them that if they were in front of me they wouldn't have seen it! Good luck!