up is often understood to mean "the entire way".
Prosecutor: Did you step onto the property?
Witness: Yes, I opened the gate and started walking towards the house.
Prosecutor: Did you walk up to the door?
Witness: No, I stopped in the middle of the walkway and went no further.
Consider the following words spoken by a teamster to his mules:
Whoa, there. Hold up!
He means for them to come to a complete stop.
Do you see the semantic relationship between "the entire way" and "complete"?
When we 'fess up or we own up to something, we give a full confession and accept full responsibility. We don't take only partial responsibility or try to weasel our way out of it.
When we man up we summon all of our courage.
And now I should probably shut up and say nothing more.