second wind

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The return of relative ease of breathing after the initial exhaustion that occurs during continued physical exertion.
2. n. Restored energy or strength.

Wiktionary

1. n. A renewed feeling of energy after a period of activity.

WordNet 3.0

1. n. the return of relatively easy breathing after initial exhaustion during continuous exertion
2. n. renewed energy or strength to continue an undertaking