I watched a presentation on a manned mission to Mars and learned it would take two years until the Earth and Mars were properly aligned for the return mission from Mars. Are there any major technological hurdles to surviving on the planet and producing the fuel necessary for the return?
One of the big hurdles to surviving on Mars long term is radiation. Mars hasn't got a very strong magnetosphere (about 1500 nanotesla compared to Earth's 65000). This is what deflects radiation and solar winds and stops it from hitting us. This is also thought to be one of the reasons Mars has a thin atmosphere, about 0.6% of what Earth has.
This means in the long term we would have to protect the astronauts and equipment from the radiation and solar winds.
Keeping the life support system running for the habitat and the space suits is the first problem. If fuel is produced there all systems necessary for that must work during the stay. The space ship(s) needed for return will need some maintenance and a lot of checks before the countdown sequence for return to earth. If a space ship for return is left alone in an orbit, it must be prepared for return after two years of hibernation in some weeks or months.
But all repairs, checks and services must be done by the astronauts themselves. They can ask the experts on the earth for advice, but nobody is there to help them. All necessary parts and tools for repairs must be there, nothing can be ordered. If someone gets sick and needs medical help, they will get advice from earth, but a surgery must be done by the other astronauts. Every medication that might be needed must be there and all necessary medical equipment. But only small and lightweight medical equipment will be there in case of an emergency.