"Argh, I have to push myself harder! I can make it!"
The runner tried, but just didn't have any more juice. He had a fantastic start around the track, but about halfway through the run, he simply ran out of juice. As much as he tried, he simply could no longer keep up the pace. The other runners who had been far behind were now passing him up as though he were standing still. By the time the race was over, he came it second to last.
There are times in life when simply trying harder will not work. Simply willing yourself to run faster will not work unless you have what it takes on the inside. This usually happens over time with good training and exercise. But this is precisely where the athletic metaphor can break down in regard to the Christian life. Consider this passage: ,
"For those who are according to the flesh set their minds on the things of the flesh, but those who are according to the Spirit, the things of the Spirit. For the mind set on the flesh is death, but the mind set on the Spirit is life and peace, because the mind set on the flesh is hostile toward God; for it does not subject itself to the law of God, for it is not even able to do so" (Rom 8:5-7).
Merely trying harder is futile. One needs the Spirit. This is why in this same chapter it says "if by the Spirit you are putting to death the deeds of the body, you will live" (Rom 8:13). It is by the Spirit. That is why we have been saved among other things by the "renewing by the Holy Spirit" (Tit 3:6). The Spirit also sanctifies (2 Thess 2:13) and strengthens us (Eph 3:16). The Spirit is "living water" (Jn 4:10f; Jn 7:37f). Maybe that is why the "Spirit, wind, and breath" are all from the same word in both Greek and Hebrew. God "breathed" the breath of life into man. Even though sin brought alienation and death, God promised he would replace dead hearts of stone in mankind with a heart of flesh and give new life through the Spirit (Ezek 11:19). God would "breathe" new, abundant life into us thorough the Spirit without measure (Jn 3:34). Perhaps this is why Jesus "breathed" the Spirit on his disciples (Jn 20:22). It is the Spirit that gives life, the flesh profits nothing.
So does this mean we are not to try harder? No. What it means is we are to walk in the Spirit and not in the flesh. However, this is much easier said than done. The flesh is our default mode. It takes ongoing transforming work to make the Spirit our default mode. The Spirit's primary tools are the word, which is the sword of the Spirit (Eph 6:17), and prayer (Jude 20). Other things include disciplines such as meditation and fasting. There is no merit in these. They will not win favor with God. Instead, these create the environment in which the Spirit works. It is like training for the race. Without it, we are powerless no matter how hard we try. This is part of what it means to walk by the Spirit.