Is 6.40 I good 1500m time?

That's good! But if you do train correctly you can do much better & you'll be surprised. I suggest you run 3-4 miles without timing it one day, just to get the exercise in. then gradually work your way down day by day until you decide to run tr mile. This Gould help your endurance alot. Make sure you do some cardio, & don't forget to stretch!

Well a 1600m is actually a mile. which is four laps. My cross country coach at our school has us sprint then slow down then sprint. We also run longer than a mile to help improve our endurance so that we can run faster, we also run up hills to help endurance. She also taught us breathing strategies so we dont get to tired. I am 14 and since we started doing that i have improved my time by 5 minutes. And i am a sprinter, not long distance. there is a girl at my school and her best time is 5:30. hope this helped.

It's probably better than I could do. Your best bet to shave off extra time would probably not be more physical endurance training (however I'm not saying this isn't good for you, keep training), but mental training. This sounds cliched and corny, but, think you can, will your body forward, and you'll get there.

Yes, it's quite good. But you should know that 1500 meters is less than a mile. 1600 meters is equivalent to a mile.