In the winter I use studded tires and enjoy riding without all the sweat. I've been touring for about 40 years off and on, but have never worried about "training"for it. Ride more, take few short loaded trips to dial in your choice of gear. Your question reminds me of a trip I took a number of years ago. I told a customer of mine I would be riding 700 miles on vacation. He said "You don't look like you're in shape for that!"
I told him, " I will be before I'm half done."

Marc