"15nm. I added all the points and route to my GPS and printed some charts. At 5knts, it would take us 6 hours to get there and back. "
Unless you have Aeolus smiling on you, or you were motoring, it didn't and won't ever happen in 6 hours if that's 15nm between the two places in a straight line. A rule of thumb is that you'll have to tack, either going or coming or both, and that means you'll be asiling at a 45-degree angle to your straight line course, so the real distance traveled will be about 1.4x the straight-line course.
I usually figure 1.5x before any current, etc. and then estimate minimum and maximum runs as long as "best guess" for a trip.
So how long did the trip take you?