NORMAN — During DeZephen Walker ’s recruitment, Oklahoma’s coaches never sugarcoated how grueling it would be to play for the Sooners. Walker is a running back from Peculiar, MO, who signed with OU in December. He enrolled at the university in January, and his first few weeks of winter workouts were different from anything else he had previously experienced. “High school is way different,” Walker said. “Here, every single day is a challenge when you're going against top athletes. Every day you've gotta be physically and mentally locked-in.” Walker chose Oklahoma over offers from Kansas State, Auburn, Missouri and Georgia. Former OU running backs coach DeMarco Murray was Walker’s primary recruiter, and he sold the young tailback on how demanding it would be to play in Norman. Deland McCullough replaced Murray in January — right before Walker got to campus. Though a different coach is now leading the position group, the grueling nature of an offseason in Norman is just like Walker anticipated.…