Two Marathons Same Result

I ran the St. George Marathon in Utah in October of 2015 in around 3 hours and 31 minutes.  I ran the Phoenix Marathon in Arizona in February of 2016 in around 3 hours and 27 minutes.  My goal in both was 3 hours and 10 minutes in both which is a 7.15 pace.  In both races I was between 7.20 and 7.30 pace through mile 18 and then that is when the wheels fell off.  I need to figure out those last 8.2 miles.