After giving NASA a heads-up via rocks stacked outside in Morse code, Mark settles in to start planning his journey. NASA has been giving him a lot of advice recently, but he'll have to figure this one out on his own. Problem numero uno: the rover needs more power.
First, he realizes that he doesn't "need the water reclaimer at all," as he can bring enough water to last him the trip (18.16). It's not much, but it's a start.
Next, he turns to the atmospheric regulator and oxygenator. He figures that he'll only need to use the oxygenator every few days, camping out for the night to give himself a full charge. He'll also grab that handy RTG again to provide heat and some extra energy.
Mark spends the following day testing the contraption, using rock to mimic the weight of his supplies. It doesn't go well at all—with the current set-up, his "sixty-four-sol trip would become ninety-two" (18.54).
Unfortunately, Mark strains his back lugging around those rocks and is forced to spend the next week inside "recovering from back problem" (18.74). Fortunately, however, that gives him tons of time to figure this whole rover thing out.
His stroke of genius is to loot some of the Hab's solar panels and backup batteries for a little extra umph. Once everything said and done, he has enough charge to travel "100 kilometers per day" (18.123). In other words, he'll be right on time.
Meanwhile, the Hermes resupply probe is set to launch in only two days.