Answer:
Limited water, food and more distance to the site.
Step-by-step explanation:
Mark's biggest challenges in trying to get to the Ares IV launch site was limited food and water for the journey and to reach the site on time otherwise his crew can't reach to him. For this purpose Watney begins the 90-sol journey to Schiaparelli, where the MAV for Ares IV has been pre-positioned. The distance is about more than three months and there is no fast moving vehicle which provide quick transportation and he also have limited water and food supply so these are biggest challenges for Mark..