I had an earlier post back in the springtime wondering if one needs to train for a metric century, or is regular commuting enough?
So I did a metric century and it worked out fine with only my 12-20 miles/day commuting as training on rolling terrain. No big hills.

Well, this past Sunday, I went for a 75 mile ride in hilly country. Didn't know if I would be able to last. But curious. So I tried.

My legs did good. They were strong to the end. I could have gone longer, but don't know how much.
My friend and I took 5-10 minute breaks every 25 miles. I think I could have gone for 100. But I would have needed at least 2 more breaks in the last 25 miles due to annoying neck pain.

Biggest challenge was dealing with annoying pains in neck and shoulders and lower back. Soulder pain came and went through ride. Lower back didn't start hurting 'till last 25 miles on some big hills. Neck started getting worse after 50 mile mark.

Nothing drastic, just annoying. I was wondering if this was a fit issue or just weakness from no training. Sitting in the same position on a bike for almost 6 hours willl make you ache, I guess.

I think you nailed it. Training might help, and working on fit might help, but that many hours of one activity has got to hurt to some degree. I find the last 25 miles of a century ride to be harder than the first 75 miles.

I find commuting to be my best training.

As for getting sore towards the end, that could be partly related to nutrition. If you don't eat enough on a long ride you can get a full body fatigue that makes everything sore. If you wear glasses, try getting a sport strap for them. As my glasses slide down my nose I find myself raising my head more putting stress on the neck.

commuting has done great for me in regards to training but..........

You hit the nail on the head in a round about way. Longer riders will really test your bike fit. For example, you bike fit might be great at 40 miles but things might start to hurt at 50 miles. Then you tweak or get a fit, and the 50 miles feels great but at 75, new fit issues arise.

So being able to work your way up in base miles is VERY important.

Shoulder pain could be from locking your elbows, which might be from a weak or fatigued core. Keep those elbows loose!

This, plus a lot of times if you're riding for longer than normal you can tend to slump with the shoulders forced back. Keeping the elbows loose would alleviate that I think. For me the key is reminding myself to relax and paying close attention to my position and posture.

Commuting is great training for long rides. However, riding occasional longer rides on the weekends will make the metrics and centuries much more enjoyable.

Congrats!

In my experience things get grim around mile 60 and by mile 70 it all washes out.

would suggest more changes in position during ride, including stretching and turning around and things like that. you stay in once position all day and you are asking for trouble.

Alternatively, you could get a recumbent.