One: food is everything. Even if you're nigh on stranded somewhere, if you have a reliable source of food you can maintain decent morale for a long time. Seals and penguins(!) seem to be very versatile in this regard: seal steaks, fried penguin, and using the blubber and skin from both as fuel.
Two: a 144' sailing vessel can carry a lot of rations. Endurance tells of the ship holding 28 men, tons of coal, 50+ dogs, three small boats, and much more. Every time in the book I expected to read that the supplies had run out, the story instead followed that the team found another 600 pounds of dog pemmican or 50 bounds of butter somewhere.
Three: leadership involves make decisions contrary to the popular opinion. In one passage the party is at Patience Camp and supplies are running low. Several miles away on the ice is Ocean Camp, which they had left days earlier, and there one could find hundreds of pounds of food stores. The popular opinion was to send a sled team to fetch it -- the food was pretty badly needed -- but Shackleton refused because of the risk: the ice between the camps was loose and moving, and one could easily fall through a crevice or mistake as solid a patch of snow over fresh, thin ice. The decision turned out fine -- Providence delivered a leopard seal later that day which provided hundreds of pounds of food for the group -- and I know it's a biased sample; I'm sure Shackleton made decisions that didn't work out so well. But the point is that being a leader means making these decisions, considering the popular opinion, but going with what oneself believes. There's a corollary: being a good follower means following a directive that one doesn't agree with. There may be nuances to a decision that cannot be easily explained in a short period of time, and one must trust leadership. (In this case, Shackleton felt very deeply that he was responsible for the safety of his team, and from his experience in previous expeditions that the risk of falling into ice outweighed the risk of possibly running short on food. That sort of intuition is hard to explain in the moment, though.)
Four: be prepared for contingencies. For the most part, this means packing wisely, bringing tools and gear you may need only on occasion. I pride myself in being prepared for most contingencies when I travel, mostly of the variety of bringing some allergy medicine or my adventuring hat. But having suitable gear can mean you can wait out your troubles.
Shackleton's journey has been billed by some as one of the greatest adventures of the 20th century. I can kind of see why one would say that but I'm inclined to disagree. The party's tale is a remarkable one but my brand of adventuring has more exploration and discovery; survival against the wild, impressive and important, is something different.