Teaching Sampler | The Journey to the Cross

For the next two weeks we are looking at "The Journey to the Cross". This week, Gene will be teaching us about how Jesus was aware of his fast approaching death as he journeyed to Jerusalem. Check out Hebrews 12:1-2:
"Therefore, since we are surrounded by such a huge crowd of witnesses to the life of faith, let us strip off every weight that slows us down, especially the sin that so easily trips us up. And let us run with endurance the race God has set before us. We do this by keeping our eyes on Jesus, the champion who initiates and perfects our faith. Because of the joy awaiting him, he endured the cross, disregarding its shame. Now he is seated in the place of honor beside God’s throne".
What exactly did his death do or accomplish? Was it just 'substitionary' - that Jesus died in our place? Then why couldn't he have just been killed when he was two and Herod tried to kill him?