Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_43728 ( "Stage" real, "Start" text, "Arrival" text, "Duration" text, "Difficulty" text )

### Question ###
How long does the one that stars with son bou of middle difficulty, and on a stage number larger than 13 last?

### Accurate SQL ###

SELECT "Duration" FROM table_43728 WHERE "Stage" > '13' AND "Difficulty" = 'middle' AND "Start" = 'son bou'