Thank you for the opportunity to help you with your question!
50 calories are consumed per mile
for 15 miles per hour, calories consumed= 50*15=750 calories per hr, this is equal to 750/3600= 0.2083 calories/second, given that, 1 calorie per sec=4.19 watt, the average power output= 0.2083*4.19= 0.873 Watts
Please let me know if you need any clarification. I'm always happy to answer your questions.