Extreme Conditioning is a Tier 3 perk from Call of Duty 4: Modern Warfare, unlocked from rank 1. It allows you to sprint much further than you usually could; roughly two times as long as normal.
This is basic but yet very effective. If you use guerrilla (hit and run) tactics, then this can help greatly. Also if you tend to get into situations where you get surrounded by enemies, this can help you make a quick escape. In later versions of Call of Duty, the perk was renamed Marathon, and in some games allowed unlimited sprint duration.