I'm currently training for a 10k with my girlfriend: the goal is to complete the race without stopping rather than any specific time. However, I'm a foot taller than she is so when we do runs, I shorten my stride quite a bit so we maintain the same tempo.
It seems to me that running 5km @ 6 minutes/km by myself vs. 7 minutes/km with her still feels like the same amount of effort by the end. Shorter strides + longer time vs. Longer strides + shorter time.
Am I imagining things? Should one of these scenarios be more work/effort, or do they equal out in the end?