I guess nobody remembers Bigelow Aerospace and their concept for inflatable space station modules.
Radiation protection is the number #1 problem with sending humans to Mars. This is according to a friend of mine who works for NASA. It would take about one year to fly to Mars. This is also why humans only stay on the International Space Station for a maximum of one year.
Being away from the Earths gravity results in bone loss, distortion of your eyeballs(near sited) and atrophied muscles.