I think you'll be fine. If you've done one double you're already over the biggest mental hurdle. The next one has more climbing, but most of it is in the first half, so you'll still be relatively fresh. As long as you keep a reasonable pace (resist the urge to go too hard too early, especially on the climbs) I don't see any problems.
You get 18 hours. I doubt the extra climbing will add 2 hours to your last ride. You had a lot of off-bike time, so cut down on that and you could definitely stay in the 15:45 range.