How did Saint Patrick walk 200 miles to get to the Irish coast? The distance from Derry to Cork is 220 miles. There is no place in Ireland that is 200 miles from the coast.
Perhaps he didn't walk a straight line? If you are seeking a ship, what good does it do to walk directly to the coast if there is no port there? If you are an escaped slave, you might have to take an evasive and circuitous route. In my opinion, there seems to be some reasonable explanations for what appears to be an inaccuracy. But even if this is inaccurate, it doesn't necessarily invalidate the entire story.
That would be true if he walked in a straight line. Consider that with winding trails and trackless wilderness to cross, he could have easily trekked 200-300 miles to cover 100 as the crow flies.