The New Living Translation has placed the idea of "readiness" as a result of having God's peace in believers' lives. Other translations give the idea of being ready to share the Good News of God's peace for believing sinners.
In one sense, then, having a clear sense of peace in your life is something you "stand upon" and supports you in your ministry. To know that you are at peace with God gives you a security to hold you during trying and difficult times in the battles you face. You'll be ready to face a very deceitful foe. This peace is that which Paul said in Romans 5:1 NLT - "Therefore, since we have been made right in God's sight by faith, we have peace with God because of what Jesus Christ our Lord has done for us."
The other way of looking at this verse Paul says we should have our feet covered with the preparation to preach this wonderful message of peace to the world. It's like putting on the appropriate footwear to either compete in sports, or work in construction, or some other specialized field. Whatever you wear should help you to be ready to do your job. And the Christian should be sure their "foot apparel" is helping them share the Good News with a very troubled world.