There's no contradiction here. The ability to move at a cost of X kj/km has nothing to do with sustaining a power output of Y kj/s.
Your body can be thought of as a directed graph of different pools of energy with different edge weights representing how fast you can move energy between pools, and your fitness as being your ability to move energy into the ATP pool. If you can walk a km in 10 minutes for 200 kj, or run it in 5 minutes for 200 kj, and you can produce 300 kj of ATP in 10 minutes, then you're going to run out of energy running but be able to walk indefinitely, even though both are equally efficient. (In practice the numbers are going to be different, and there are more conversions than just glucose to ATP to think about, but I think the example illustrates the point).
Your body can be thought of as a directed graph of different pools of energy with different edge weights representing how fast you can move energy between pools, and your fitness as being your ability to move energy into the ATP pool. If you can walk a km in 10 minutes for 200 kj, or run it in 5 minutes for 200 kj, and you can produce 300 kj of ATP in 10 minutes, then you're going to run out of energy running but be able to walk indefinitely, even though both are equally efficient. (In practice the numbers are going to be different, and there are more conversions than just glucose to ATP to think about, but I think the example illustrates the point).