second wind

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A renewed feeling of energy after a period of activity.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the return of relatively easy breathing after initial exhaustion during continuous exertion
• noun renewed energy or strength to continue an undertaking