I've read a few times about how cross-training when injured is a good way for a faster runners to keep aerobically fit -- but it's not a substitute for running.
I was thinking about how this could affect slow runners, like your typical 20-30 min 5k hobby jogger or 20 mpw runner. Of course, this theoretical runner won't be losing weight as a result of this training -- imagine they have a reasonable BMI in the first place.
Anything goes: cycling, weights, stepper, swimming, jazzercise, etc.