I am a fan of using the bike, elliptical machine and rower as tools to keep the knees and legs healthy for a after running.
There are a few specific training programs I have used:
BIKE(in the gym): Ride 15-30 minutes with clips; spend 2/3 of the time Pulling through the stroke. [lift on the upstroke but do not push down] this exercise will strengthen support muscles that are useful / balancing during runs.
Cross training is not directly useful in improving your performance BUT it balances the effort and allows you to keep going longer and healthier.
ELLIPTICAL: One of the major benefits of the elliptical trainer is that you can go backwards / reverse- and do it safer than on the road or track. backwards running is a great tool for runners struggling with or trying to prevent ITBand / shin splint / sciatica issues.
Once you are comfortable with the equipment start using regularly and do a workout with with 60-75% in reverse and at a variety of resistance / elevation levels.
Some info on elliptical trainers.