Why are the ponies so tired near the end of the journey?

Here is the quote,

"It was on May the First that the two came back at last to the brink of the valley of Rivendell, where stood the Last (or the First) Homely House. Again it was evening, their ponies were tired, especially the one that carried the baggage; and they all felt in need of rest. As they rode down the steep path, Bilbo heard the elves still singing in the trees..."