So, I'm looking for some reassurance that the 100 solo miles aren't junk miles in terms of training (they certainly aren't mentally but that's a separate
issue).
The term "junk miles" is only meaningful in the context of already having a program or training schedule that results in a
defined goal.
So as long you realize that your 100 mile rides are for no other purpose but to make you good at riding 100 miles then thay aren't "junk" - they are "base miles."
However, if you have alternate cycling goals, such as getting faster and keeping up with you wife, or climbing a hill faster, then you have to pay attnetion as to how your 100 mile rides affect the other kinds of riding that are "goal oriented."