There are so Many thoughts/Options/opinions on bike 'Fitting' out there, especially the internet, you'll surely be able to have a good choice on what to use.
I've given advise here before, and honestly, it's very difficult to give this kind of advise online, without personal contact with the rider... So, I learned to not try, beyond basic concepts...
SO, use any of the methods noted in this thread or any you feel/assess as being a good method.
But things to consider
Have you been riding for some extended time or is this a 1st real 'fitting' or after a very long hiatus?
If you've been riding fairly regularly and now think you need adjustment, consider what changes you need to address.
Then, Make only one small adjustment at a time, ride/try for a while, repeat.
Hand numbness - If you primarily ride with your contact point at the bars has the center channel of the hands near the wrist; that will accentuate hand numbness - find better hand positions.
Locked elbows posture will accentuate shocks into the hands, up the arms to the shoulders, neck and upper back. Bend your elbows when riding. This is a problem I see with more than 90% of those I see riding bikes... If you're on a laid back cruiser style bike, very much less of an issue. But with a road type bike (or similar) bending the elbows is key to preventing pain and injury.
There was a thread on BF about numbness and padded gloves - one suggestion was 'Sting Pad Pro' on amazon. I tried it, it works ! I have a pair. My wife uses a pair, she loves them.
Fried Glutes
You say '1st 100K ride', in how long? If all your rides have been well below that length and ride time - then what would you expect?
A more forward position will engage the Quads more... (depending on where you were before).
SLowing cadence as you tire in a ride, will have you putting more force into each pedal stroke, if you try to maintain the same pace.
Add miles in a reasonable increase level... If you commonly ride 20 mi (or whatever) then ride 30 mi for while. If you do 40 mil rides frequently, a 60 would not be a reach... Age counts here, Fitness counts for any age.
I ride almost every day, and my average ride is now 30 mi. But I mostly do 35 ish mi. and at least 1 day at 45ish mi. Then some shorter 23-24 ish miles on recovery days, which brings the avg down to 30.
I would find riding 60mi here, solo, to be a long day, cause I would also prolly need to do 4000 ft min.-5000 ft gain to get that, where I live. I can relatively easily do 60 mi. riding in a group...

'Fitting', as far as my opinion, is about fine tuning - unless you're currently position and posture is Way Off. A Fitter can help. But they do not have real experience and understanding of you, so it often is a multi-session effort. They are not like a 'coach' who have good fitting experience.
Honestly evaluate yourself and your riding position and habits. If you're not able to do that, try something else, like a 'fitter'.

Ride On
Yuri