Grace is disorienting. We see this several times throughout Scripture. Isaiah has a similar reaction when he gets a view of the throneroom -- he thinks it is his end. When we get a glimpse of the holiness of God, we cannot help but be humbled and feel unworthy.
This is the beautiful story -- we are unworthy, but Christ, by his blood, makes us worthy to stand before God. It's all a free gift. Simon, James, and John recognize the immensity of the gift, and they follow Jesus, wherever he will lead. I don't think they fully grasp the next steps, but they grasp the power and holiness of Jesus, and that is enough.
In the same way, we often don't understand all the next steps that are before us. But when we realize the perfection and love of Jesus, we can be emboldened to follow, trusting in God to sustain us along the way.