Try this one:
select * from MyTab T where date_add(T.runTime, INTERVAL 20 MINUTE) < NOW()
NOTE: this should work if you're using MySQL DateTime format. If you're using Unix Timestamp (integer), then it would be even easier:
select * from MyTab T where UNIX_TIMESTAMP() - T.runTime > 20*60
UNIX_TIMESTAMP() function returns you current unix timestamp.