Sometimes I feel like it's less about the time you spend in the saddle, and more about the recuperating time you need out of the saddle. I know I can ride 10 or 20 miles at a stretch, chill out for half an hour, and do it again. I know I can ride 50 miles between breakfast and dinner, relax all evening, and wake up ready to do it again. If a night's sleep isn't enough time to recuperate, then at some point a multi-day tour will weigh on you. For my part, within the confines of available daylight, I try not to tax myself throughout the day, so that I'm not wiped out at the end of it. Breaks to enjoy the scenery, long meals off the bike, whatever it takes to make sure that it's a ride rather than a grind. If can do 50 miles, sleep well, and wake up ready to go again, then you're in a sustainable pattern.
But it doesn't sound like endurance is your number one issue. Saddle sores are not something you build up a resistance to. They're something you hopefully learn to avoid, and when they happen, you learn to treat them and avoid them in the future. A lot of what you're talking about sounds like you're taking steps designed to prevent sores from becoming infected, which is a good idea, but it's not getting you closer to real goal: finding the right shorts and saddle combination so that you don't get sores and maybe some creams to prevent chaffing before it becomes a problem. Getting your legs capable of spending days riding is a matter of practice, training, and endurance. Getting your butt ready is a matter of the right equipment.