The truth is that high level sports is about specialism. I was a little surprised that Murray said he ran close to a 57 second 400m all out (which is pretty poor) - meaning his capabilities at 400m have probably already dropped significantly from his likely capabilities at 100m. The 5k might be even more surprising.
Wow 57 second 400m is pretty poor, considering Murray is suppose to be one of these in incredible shape ATP players. Heck I ran a 57 s 400 m when I was 14 years old and was the slowest guy on our freshman 4 x 400m relay team.