And not a sprint, then the application probably isn’t, “pace yourself”, or “keep going!”
The application might be: you need to accept the reality that you are currently physically incapable of actually doing this. Direct effort, or positive thinking, or all the encouragement in the world won’t be enough for you. You need to undergo a disciplined training regime, adjust your diet, consistently push yourself over time, and get into a state whereby you can actually do it. Day in, day out, embracing processes that you can already do, that will eventually enable you to perform feats you can’t currently.
Disciples through the ages have found these disciplines absolutely necessary in terms of sustaining a walk with Jesus over the long term. Grace is opposed to earning, but not to effort. He’s wanting to change you into a kind of person, and we’re not expected to be passive in this process.
Disciple, ‘Running to Win’ means you’re going to have to train.