Use the verb rally to describe that last push to finish a difficult something. When you hit that last mile in the race and are so tired you want to quit, that's when you rally, finding the strength to pick up the pace.
The verb rally is often used to describe a physical effort that has been renewed, but it can describe anything that involves bouncing back from a setback. For example, if something makes you feel disappointed, you rally to get your positive attitude back. Rally can also describe bringing people together, often to support a cause. The event itself is also called a rally, like pep rally in which students boost the confidence of their team as it heads into the championship game.