Answer: Mihaly Csikszentmihalyi would describe these moments for Charlene as her undergoing the "Psychological Process of Flow"
Explanation: Mihaly Csikszentmihalyi is a psycologist from Hungary that identified and formed the Concept of Flow also referred to as being "in the zone"
According to the concept, a person carrying out an activity becomes fully engrossed and focused in that activity that he or she begins to enjoy it and gain fulfillment from it.
The activity referred to here is usually one with a set goal and feedback on the person's performance.
Here, the Flow Concept describes Charlene feeling because;
1. She has a set goal (training for Iron Man triathlon) and
2. She most likely has gotten feedback for her performance (because it's physical exercise, the results on her body would be visible).
So, Charlene's practice exercise for her triathlon has become fun and not just work because she has began to enjoy it and find fulfillment from it.