I’ve biked 100 miles+ in one day twice in my life.
Once was on a supported charity ride in 2011 - I went 102 miles on a hybrid.
In 1993 I biked 103 miles in a day on a vacation biking across Virginia. That day I went from Fredericksburg to White Stone. That was on a road bike (of sorts - a Schwinn Continental).
I’ve also done 75 milers, 60 milers (“metric century”) and 50 milers a few times, but those two are my only “centuries.”
If you can run a marathon you can run a 50K. It’s just a matter of keeping your pace steady and don’t think about how much distance you have to cover. I rarely race with headphones but I found them really helpful in the second half of my 50K. It kept my mind wandering which was exactly what I needed.
And keep just a little in the tank so you can look good crossing the finish line 
I didn’t finish that loop but my training did help me hike more than 20 miles in Breckenridge in a day and not feel particularly tired afterward. I looked tired on the trail because people kept passing me on the way up and some even asked how I was doing and I didn’t want to tell them that I had already walked almost 20 miles before noon. I caught up with them downhill because I wasn’t too tired to let gravity assist me downward where it was safe to do so. I find that slowly jogging on a downward slope is as or less tiresome than walking.
Holy shit, just went back and read my post from Sunday morning…apparently I was fucking pumped, because that’s more exclamation points than I care to take credit for. (!!!)
Anyway, it ended up pretty good…I ran a rail-to-trail south of Ottawa, KS. Parked in the middle and ran north then back, restocked at the car, and then ran south and back. Finished 26.2 in 5:25, which is slower than the pace I was shooting for but still acceptable. Gives me an hour and 35 minutes to finish a 50k before the 7 hour cutoff
I ran comfortably, walked 1/10 of every mile, and didn’t lose my breath or push my heart rate until the last 7 miles or so, when I realized I was going to be just fine and picked it up. Very comfortable with the prospect of piling on 5 more miles in two weeks.
You’ve got the distance, 5 miles is nothing after that. You’ve got the pace down pat. Work on getting enough sleep, keeping your legs loose and fresh, and eating right.