How should I safely push my hiking distances to the next step up?

I'm a casual hiker. The longest hike I've done has been 13 miles. There's a hike that has caught my eye, but it seems to be around 17-19 miles depending on the route you take, and quite difficult. I'm trying to figure out a good way to determine if I'm ready for it or not.

I've done lots of 9+ mile hikes without trouble. I was pretty tired after the 13 mile hike, but it was also in pretty terrible conditions (raining the whole way, lots of elevation gain, had to walk in a river because it was flooded, had to run a couple miles to get back before dark because I started too late in the day).

What I want to avoid is getting 15 miles into this hike and realizing "huh, I can't finish this before dark...". Any tips for figuring out my limits? I'm also trying to be extra cautious since it would be done solo.