At the risk of being redundant:
Saddle
I found the type of saddle to be less important then posture. Some Clydesdale bikers sit upright with all the weight on the saddle. That will hurt. I bent forward where the bars are a little below the saddle. That puts pressure on your wrist and hands but is good for your behind. Another trick is to get off the saddle often. Either with a rest or better learn to pedal while standing up. I have trained to stand up for 5 minutes or more.
Good shorts will help. And there are good saddles also. I use Terry Touring and bike in street shorts without padding often.
Nutrition
You will feel miserable on longer rides if you do not eat right. Many of us are trying to loose weight while biking. That is a problem. A good breakfast followed by Gel with 200 calories per hour is good advise. Gatorade helps also.
Training
I had your type of issues until I started training harder. My standard is 50 miles per day and 70 miles/day in Florida in the winter.
Of course I am retired.
