I would suggest you look for Marc's COD pack. It has alot of great scripts for PB, but here is Marc's script for what you are asking for.
;Header
desc = Sprinting
;Triggers
;Make the player only be able to sprint a certain distance before running out of breath.
;Change the plrspeed settings to adjust speed.
;Adjust stamina variable to increase or decrease player stamina.
;Script by Marc Steene from the Modern Warfare Script Pack
:state=0:dimvar=stamina,plrspeed=100,setvar=stamina 100,etimerstart,state=1
:state=1,plrusingrun=1,vargreater=stamina 0:subvar=stamina 1.5
:state=1,plrusingrun=0,varless=stamina 100,etimergreater=100:addvar=stamina 2,etimerstart
:state=1,vargreater=stamina 4,plrusingrun=1:plrspeed=100
:state=1,varless=stamina 5,plrusingrun=1:plrspeed=60
:state=1,plrusingrun=0:plrspeed=100
;End of Script
Just so you don't have search for it and I want Marc to get full credit for this, here is the link for the rest of the scripts.
http://forum.thegamecreators.com/?m=forum_view&t=162837&b=24
