You can not tell your coach he is an idiot, so you must do the training necessary to run that fast, which means you must do intervals at race pace. Do a day of 6 to 8 times 400 running at just under 90 seconds, walk 400 to recover after you are in better shape you can walk 200 for recovery.
Another day you should do 4 to 5 times 800, under 3:00 with 400 walk for recovery.
This will get your body in shape to run at that speed.
Improving your mile time by 40 seconds will take some work but it is possible. If you don't have a watch get one and calculate and time your splits for each lap or even 200m to make sure you are on pace. Try doing some long runs to improve your endurance as well. Since you are a sprinter try not to wear yourself out too fast and really work on pacing yourself.
Believe in your self it really helps if you keep doing all the right things like having a proper diet, training enough and be sure to push yourself at training, get enough sleep. And keep telling yourself that you can do it eventually it will happen! Good luck hope it works out for you:)
Believe in your self
go rock climbing it will get your legs shredded
