What analogy did the Receiver compare his life to?

The Giver compares his life to going downhill in the snow on a sled.

It's as if…" The man paused, seeming to search his mind for the right words of description. "It's like going downhill through deep snow on a sled," he said, finally. "At first it's exhilarating: the speed; the sharp, clear air; but then the snow accumulates, builds up on the runners, and you slow, you have to push hard to keep going and –"