Basketball is more about quickness and short -- very short -- bursts of speed. I think basketball players, especially smalls, are great athletes but you're talking about them doing something athletically that they're very unused to -- running in a straight line for what for them is a very long distance. I think most bigs like Lebron would have a hard time sprinting 100m without falling apart because basketball courts are only 100 FEET long. You can't even get up to top speed in that short a distance. Basketball's more about short sprints and changes of direction and of course, jumping, than continuous running. Unfortunately, there's no track event that combines very short sprints with quickness. They'd probably be excellent at shuttle runs.
With just basketball training I think Lebron would have a hard time breaking 60. He has the speed but doesn't have the strength to sprint 400m. Just the unfamiliarity of doing that would really hamper him IMO.