He had known they ought not linger. He had known they ought to leave. But he had not been the captain of their party, so he had not pressed. And now, Boromir was dead, Frodo and Sam vanished, Merry and Pippin taken. The ache turned into something else as they prepared Boromir's body for the boat, as they gathered what supplies they would carry, as they turned away from the Anduin and instead headed toward Rohan, in pursuit of the Hobbits. The ache became a biting, tearing pain, and Legolas imagined his father running pace beside him. Were you raised to follow the commands of a mere Ranger? Did you not trust in your own instincts, your own knowledge? Boromir is dead because of you, because of your lack of resolve. Your complacency. Your placidity. Merry and Pippin have been taken because you could not keep them safe. Frodo and Sam cannot succeed, not alone, not in the wilderness. The quest will fail because of your shortcomings.
Running, hours and hours, that voice a relentless drumbeat in his mind that hammered with a steady pace. If Legolas had felt any simple physical pain from this work - tired muscles, complaining joints - he would not have felt it over that voice.
He already knew he was changed from this time in the Fellowship, much as Tauriel had been changed by her time with the Dwarves. It was why his people were leaving these lands; the cup of Men and Dwarves, the violence and conflict with which it was filled, was not for their lips to drink. The biting pain shifted form one final time, into a pressure inside his chest and behind his eyes. Each footfall came heavier, each step grew that much harder, and Legolas felt despair for the first time in his long life.
Then, as the sun rose on the second day, and as they slowed just enough to steal bites of lembas from their packs, Aragorn clapped a hand on Legolas' shoulder. A look passed between them, and he saw on Aragorn's face that same despair, that same ache that came from knowing that one was partly responsible for great grief. Aragorn understood that Legolas hurt, down to the very core of his spirit, because Aragorn did as well. A hand to his shoulder, a glance that lasted three steps, maybe four, and then they continued.
It was hours before Legolas realized that the ache, the pain, the pressure was gone. There was a purpose yet before him, and this time, he would not fail.