Wow !
I think you are doing great. I remember like it was yesterday when I could actually ride 10 miles it was almost as exciting for me as finishing my first century ride.
You want to build a big base with miles and add to that base endurance by challenging yourself on hills and picking up your average speed by doing intervals. Well that's how I did it, years ago and I'm back at it after a couple of years off.
Now I'm doing good to do a 10 miler but the last one I did I felt pretty good after, not my usual tired self. It all takes time and mostly time ON the bike!
Hang in there you've gotten a lot of good advise and will do great I'm sure we'll be hearing stories about your first 100K soon!
Life is like a 10 speed bike, we all have gears we never use.
Charles Schultz
"The bicycle is just as good company as most husbands and, when it gets old and shabby, a woman can dispose of it and get a new one without shocking the entire community." — Ann Strong, Minneapolis Tribune, 1895