To sum up my reaction to "Marooned" in one word, it would be "annoying." The main reason I found your book annoying was because I'd been kicking around a similar storyline - time travel to Carbondale circa 1969 - and you beat me to the punch.
I enjoyed parts of it, especially your descriptions of Carbondale in the early '70s. The story picks up whenever the narrator, Old Pete, gets out of the dorm (and his head) and heads to the strip or the radio studio. You do a good job of describing hangouts and bars, but Old Pete spends way too much time pondering chipped tiles, clouds, his sphincter and daily affirmations. He's a prude who doesn't smoke pot, feels guilty about drinking, and doesn't even attempt to pick up chicks, choosing instead to obsess over his algebra class. I'd think a 58-year-old guy given a 20-year-old body and a second chance at Beaver City during the hippie era would first and foremost want to get laid. He'd also want to get stoned and party like it's 1971 (at least the reader would!).
To me, Old Pete seemed not just politically apathetic but oblivious, with few insights, curiosity or passion as to what the students were rioting about. Although it is there as a backdrop, the student power movement of the Vietnam/civil rights era seemed to be of less import to Old Pete than a chipped tile and an algebra test.
When Old Pete rescues Catherine from the riots, he ponders what a mature person would do, but instead of taking her to the safety of his dorm room, where she could call her parents, and he might have had sex with her or proposed marriage, he instead walks her through the teeth of the riot, then abandons her at her car so she can drive home alone. I don't think either Pete would have done that.
Another problem with "Marooned," for me, is that the first time Old Pete forgets to shave his mustache, the reader knows it's because of Young Pete, yet it takes Old Pete all the way up to the climax to figure out what the reader has known from the get-go, so the climax is kind of anti-climatic.
It seemed to me an odd choice to make the protagonist and antagonist unaware of each other, so they never interacted, much less had a meeting of the minds, so to speak. Couldn't Old Pete have used his magic pencil to scribble a note to Young Pete, regarding choices and consequences, and Young Pete could have written a return note on the bathroom mirror about chilling out and going with the flow, or whatever? In the tug-of-war between Old and Young Pete over Catherine vs. Tammy, their attempts to influence each other might have had more pathos, and possibly character growth - one learns something from the other - if they'd discovered each other, rather than a sudden blackout, deus ex machina, Young Pete proposes to Tammy, the protagonist loses to the antagonist.
Well, as warned, my comments were pretty negative, but mostly had to do with what isn't in the book rather than what is. And I did care enough about the characters and story, as well as setting, to read it all the way through. So congrats on writing a book many people should enjoy.
I on the other hand, thoroughly enjoyed reading "Carbondale After Dark."