The single factor that determines how far or how fast I run is not my heart rate or my leg muscles. It's not even the heat or the hills (We call 'em that here in Illinois just to humor the natives. I'm from Eastern Kentucky. I KNOW what a hill is.) It's not lack of hydration or my shoes that causes me to take a walking break.
I knew breathing would be important, but I didn't realize what a make-or-break you part of running it was. Once the newbie, gasping phase passed, I found there's a rhythm to it. That steady in and out increased my endurance far beyond what I believed I was capable of.
In the New Testament, the same Greek word is used for breath and Spirit, and there's a definite parallel between physical running and running the race God lays before me. The key to success and endurance is learning to run in the rhythm the Holy Spirit dictates. The "heat" and the "hills" are what God chooses. I picked up my shoes already. But breathing gives strength to my legs to keep those big muscles moving.
If I push too hard, my lungs can't keep up and I get that familiar pain in my side. In the same way, if I try to push beyond the Spirit's direction, He'll give me that same hitch as a door closed, an obstacle or sometimes a spectacular failure.
In Matthew 11:29 The Message says "Walk with Me and work with Me–watch how I do it. Learn the unforced rhythms of grace." Breathing. In and out. The Holy Spirit. In us, then out through us.