I was curious about this as well, and the abstract from the study said:
"voluntary wheel running is characterized by rapid pace and short duration, whereas forced exercise involves a slower, more consistent pace for longer periods of time. This basic difference between the two forms of exercise is likely responsible for their differential effects on brain and behavior."
It also did say that both groups ran the same distance, but the non-forced rats did it in more sessions and a faster pace.
I think that in order to understand the nature of "forced" vs "voluntary" in this instance, you may have to consider how the rats normally cover distances. Humans, unlike most mammals, are designed to run for longer periods of time. Sorry if it's already been said.
"voluntary wheel running is characterized by rapid pace and short duration, whereas forced exercise involves a slower, more consistent pace for longer periods of time. This basic difference between the two forms of exercise is likely responsible for their differential effects on brain and behavior."
It also did say that both groups ran the same distance, but the non-forced rats did it in more sessions and a faster pace.