The 'light tread' refers to how the poet would walk freely some nineteen autumns ago. Then, he was a free man with no worries and would go around for days wherever he wished to, the same way the Swans did. But now, he has a 'sore heart', which indicates a slight pain in the heart. At the same time, when looking at the Swans moving around freely and carefree reminds him of the responsibilities he has been tied to, and he has a lot to worry about. He feels he can no longer walk around like a free soul. Neither do his physical or mental conditions allow him to do so.
What is the Planning Process?