ST. LOUIS — To understand Gregg Marshall is to understand that he’s a product of his past.The leader of unbeaten Wichita State wasn’t born into coaching with a silver spoon. He didn’t learn at the knee of Bob Knight or Mike Krzyzewski, or lean on a resume line that might read “UCLA” or “Kentucky” to climb the coaching ladder.No, Marshall was a skinny guard who had to scrap and claw just to make it on the floor at tiny Randolph-Macon, the stately Division III school in Virginia that eventually gave Marshall his modest start in the business, doing as much laundry as scouting reports for the late Hal Nunnally.From there, Marshall’s career took him to Belmont Abbey in North Carolina and the College of Charleston in South Carolina. He spent time at Marshall and finally became a head coach at Winthrop.These aren’t the kinds of places that tend to produce Final Four coaches.