No it's not mysticism. It's simply a method of remembering God in simple ways moreoften. It's part of praying without ceasing. It's a way of consistently bringing our mind back to whatever truth The Lord has brought you to for a season. My pastor prayed about what his would be and the Lord told him it was a season for him to learn to listen better. Thus throughout the day he whispers "I'm listening."
As I prayed about mine, in all my type a glory I had found several very practical and sensible words and phrases, but none of them quite felt right. One day as I was riding the bus to work, we came upon some heavy traffic. In my spirit I sensed something was wrong. I never saw anything except for a few cones queen we finally passed it. But as we were waiting the word "Jesus" sort of spontaneously leaped out of my mouth. All the people stared at me on the bus and I blushed. "It was the only thing I could think to say." However it was a prayer more than an exclamation. After that time I found myself either outloud or in my head saying "Jesus" as I realized I needed Him.
Jesus repeated multiple times "...not my will but yours, Lord". That is a breath prayer.
I understand that certain people have their issues with Rick Warren though I do think its far fetched to lump him with Oprah. However, this is not mysticism, Rixk Warren, or Oprah based. It actually came from hesychastic prayer and was modeled after the psalms where one phrases was repeated to remind you of an entire psalm. The most well know breath prayer is "Lord Jesus Christ, Son of God, have mercy on me, a sinner."