Where are you?
I ride from Portland to Eugene, and Eugene to Portland a few times a year. You're welcome to join me, varying from about 140 miles to 200 miles.
Whew, 15 hour time limit for 154 miles. Actually, that is averaging only about 10 MPH (including restroom breaks, rest stops, etc). And, with some luck, you'll not have to carry much of anything with you.
As far as speed, there are two things. Speed and endurance.
I can hit 20 MPH for say 10 miles. But, I just can't keep it up. I find that I'm slowing down considerably as I get over 100 miles or so.
I can't motivate myself to do a lot of intervals, but you presumably have to work on both short term effort & quick recovery from hard efforts (intervals will help with that), as well as overall endurance and fatigue. Really, I think the more century rides you do, the better off you will be.