#1 nevr had P+N from cardio.
#2 i would keep running at any pace [preferably lower than 80%MHR] until you can nail 14-15miles [always better to over stretch the distance in prep for a race i find] then work on bringing that time down.
also, don't listen to morons who tell you to find a pace and stick with it all the way through. its crap. IME, your best bet is this: 1st quater-slow and easy, to get warmed up and setteled in, 2nd quater-quicker still, 80-90% of your pace in 3rd quater, 3rd quater-bomb it, 4th quater slow to pace of 2nd quater and save some for last mile, final mile-sprint
hope that helps mate