Congratulations on getting ready for the 15K and good luck at the race! It's a somewhat unusual race distance, but it's a good challenge if you've been doing 5K's and 10K's. I do mostly 5/10K's but have done three 15K's for that reason. I'm a little slower than you (1:23 for the 15K). Exactly how much water you need will depend on the race conditions (temperature etc.) and how well hydrated you are before the race. I did three water stops on each of the 15K's, typically around miles 4, 6 and 8. This was in cool, moderately humid race temperatures (Coronado, CA, on the coast) and a flat course. To calibrate, I normally don't stop for water on a 10K unless it's unusually hot.

Because the 15K is not a common race distance, you may find that the overall field is somewhat faster than your typical, say, 10K.

Congratulations on a strong race and a good finish!

You can also look at it this way...a half marathon is only 3.8 more miles than a 15K...you're already within striking distance!