Final answer:
Endorphins are the likely reason why Melissa feels little fatigue or discomfort after running a marathon.
Step-by-step explanation:
The release of Endorphins is probably the reason why Melissa feels little fatigue or discomfort after completing a marathon. Endorphins are hormones that play a major role in inhibiting pain and are released during vigorous exercise like running. These hormones contribute to the positive effects of exercise on anxiety and depression, and are often associated with the phenomenon known as a 'runner's high.'