I spent nearly an hour on the phone with Brad Barton on Saturday, the day after his mind-blowing 4:19.59 M50 mile at Nashville — taking a huge chunk out of Nolan Shaheed’s 2001 record of 4:25.04. My first question for Brad: Where have you been for five years? (I had forgotten about his pair of 1500 American records in 2016, including one at the Olympic Trials masters exhibition.) But seared in memory were his incredible 2013 and 2014 seasons, where Brad dominated distances from 1500 to 3K steeplechase, setting a slew of records — duly noted in my new story…