What's important about Mr. Harvester (who actually doesn't have a name when Adam meets him on his bike ride)? Well, for starters, he's the first person Adam meets along his journey. Also, he's described very specifically: he has a loud, booming voice; he calls Adam "Skipper"; and he has "a face with so many red and blue veins that it resembles the road map in his hands" (3.9). These memorable images might serve less to help us understand this minor character than to keep him in our heads. Thus, when we get to the end of the book and Adam has arrived back at the hospital, we immediately know that Mr. Harvester – the old maintenance man who calls him Skipper, has a loud voice, and veins like a map – is the same guy he met on his journey. And it's at that moment that we know something is up with the bike ride.
Adam meets Mr. Harvester at a gas station, where the old man seems to just be hanging around, working on passing cars. In addition to talking to him about his trip (the old man gives him advice on his journey), he also warns Adam not to trust anyone he meets. Overall, he's a bit dramatic, almost as if he's playing some sort of game with Adam (which maybe he is).
One last bit of information that we get from Mr. Harvester is that he had a son who died at Iwo Jima in World War II. Iwo Jima was one of the most brutal WWII battles (there's a memorial in Washington, DC), and it took place in 1945. This is the first time in the book that we have a specific time reference, and it tells us that the book takes place when it was written – the late 1970s. (If the old man's young son died in 1945, it works out that Mr. Harvester would be an "old man" in the '70s).