When it comes to exercising, or being good, or doing anything I don't want to do, I think of the parable Jesus tells of the two sons:
A man had two sons, and he came to the first and said, 'Son, go work today in the vineyard.'And he answered, 'I will not'; but afterward he regretted it and went. The man came to the second and said the same thing; and he answered, 'I will, sir'; but he did not go.
"Which of the two did the will of his father?" They said, "The first." Jesus said to them, "Truly I say to you that the tax collectors and prostitutes will get into the kingdom of God before you.
It doesn't always feel good doing what you don't feel like doing. It makes me grumpy, cursing under my breath. Talk is cheap, though, so it doesn't matter much what you say. Just do what God asks of you. Best to do it joyfully, but even if you can't muster that, just get the job done. If you're talking about quitting smoking, quit. If you're wondering whether to bag the run today, don't. It might hurt, or at least be an inconvenience, but at least you'll have a clear conscience at the end of the day. Make your yes mean yes and your no mean no. But when they don't match up, go Nike and just do it.