This is an interesting study, but I'd like to see it with four control groups: Exercise + mental stimulation, Exercise with no mental stimulation, no exercise + no stimulation, and no exercise + stimulation.
Why not throw in 'social stimulation' as well? Both rats and humans are very social creatures. It'd be interesting to see how an isolated, well exercised rat's brain compares to a socially active, non-exercised rat's brain.
anecdote : being very reclusive, going to a social event will weigh on your brain, so much that a night feels like a week. So much randomness compared to your own routine. Exhausting but not far from what your body feels after a good workout.