A few hurdles
Getting a group of people to Mars hale and healthy will be a bigger achievement than coming up with a way to liberate Oxygen and make fuel in-situ. The human body deteriorates when in zero G for extended periods of time. When ISS astronauts return, they spend a long time getting back into shape even though they work out while they are in space. A person on a Mars mission is going to have to hit the ground running from day one with no on-site support staff to help them with health issues. If things don't go exactly to plan, broken bones, death of some of the crew or other health problems, the healthiest of complement are going to have to work even harder.
Mental health will be an issue as well. For anybody that has seen an Apollo moon capsule in person knows just how tiny they are. A Mars crew capsule will have to be bigger, but the space will still be less than a budget New York apartment. With no way to avoid crew mates, the team better be highly compatible. NASA has studiously avoided confronting the inevitability of sex among space travelers, but they will have to sort that out before a Mars expedition. Is it best to send couples? Married? A "pan-mixia"? or a single sex crew? Pregnancy and children better be addressed. It's not known if a child will develop properly in 1/3 G. Mice/rats and other studies can provide some insight, but nothing will be known conclusively until the final human trial and it could have a sad outcome that needs to be anticipated.
People will be the biggest challenge in a Mars Mission and a small base on the Moon first will teach us a lot about how many assumptions are being made in both hardware and meat-space with a good possibility of returning a crew to Earth on short notice or being able to send needed supplies quickly.
Biting the hand that feeds IT
