Depends what track and field event you are talking about. But the fastest man in track in field is Usain Bolt. He ran the 100 meter dash in 9.69 seconds setting a new world record.
9.72 seconds by usain bolt
the record for middle school boys 1600 meter run is 5.55.1
Usain Bolt holds the world record at 9.69 seconds for the 100 meter dash. Usain Bolt holds the world record at 19.30 seconds for the 200 meter dash. Michael Johnson holds the world record at 43.18 seconds for the 400 meter dash. So I would say Usain Bolt.
If you can trust the person who told you it's a 400 meter track, then no calculation is necessary. The length is 400 meters.
1:41.11 by Wilson Kipketer in 1997
Usain Bolt 9.58
There is no record for 11 year olds for track