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.
Any aerobic activity will increase aerobic capacity
I got from 25 min 5k to 20:30 exclusively by cross-training (biking and swimming). Legs (running specific muscles) definitely become the weak point then, and HR is very high due to inefficiency/bad running economy. But it definitely works to get faster up to a point.
At my current level (low 16 5k) cross-training doesn't seem to have any significant effect anymore, but it can lead to overreaching if done too much in addition to running.
Triathletes can get pretty fast with cross-training, but they also put in incredible amounts of hours.