First consistent training is key, so if running 100 mpw ruins consistency over time it is not an option.
Then, eliptical training has transfer effect to running, so any added training with transfer effect should give improvements. Not all alternative training forms have a transfer effect.
Another possibility with low-impact alternative training is if you could not only add milage, but also at a higher intensity. I would not do that in step 1, but intense running has a large recovery need due to impact, something low-impact training forms has much less of. That is why cyclists can do much more threshold training and I would have experiemented at a later stage with adding more intensity to the eliptical.
Go test with steady progression of course...
I use cycling as alternative. I can do tougher running days, even double workouts, since I recover with cycling inbetween. Cycling has also increased my strength and general conditioned my legs to be less injury prone. More cross-fit sort of...