Haha, well depending on the exact ship they go on they'll probably have substantially more room than the ISS. That was the main motivation for things like people staying 370+ days on the ISS. And long before the ISS even existed, the USSR was also actively pursuing this. In 1988 Valeri Polyakov stayed aboard the Mir Space Station for 240 days. His first words after landing were, "We can fly to Mars." [1]

After that he spent a whopping 437 days on Mir (which had about 1/3rd the pressurized volume of the already claustrophobic ISS) to see how the human body would respond to long-term duration in minimal gravity. Upon landing back on Earth this time he decided to get up and walk from the capsule to his rest point (astronauts are normally carried/rehabbed due to muscular atrophy + dysfunctional balance/orientation, even for far shorter stays), making a point of the fact that he was just fine. Dude was just a complete badass. The USSR would have beaten us to Mars if they hadn't collapsed in 1991.

In any case, it's probably a good idea to do a flyby because there will be, with near 100% certainty, some thing things we hadn't considered and others that we simply were not aware of. By first doing a flyby and then a landing you increase the chances of success. And the people doing the flyby will probably be mostly the same people doing a landing a couple of years later - so it'll be more like "See you soon."

[1] - https://en.wikipedia.org/wiki/Valeri_Polyakov#Cosmonaut_care...