Biking and swimming are obviously things necessary for triathlon but they are not a substitute for running. When race day comes you need to run too. If you are limited in how you can train for running, you need a better substitute.

Given all the alternatives people have posted here, I would rank them as follows

1. Running on dirt/trails -- absolutely.
2. Running on a treadmill - boring yes but certainly lower impact than pavement.
2. Aqua jogging - The best non-impact substitute there is. Definitely worry about form and do not use the flotation vest
3. Walking - not just up hills but everywhere. Combine this with aqua jogging if you can.
4. Elliptical - a poor substitute

[edit] as the post immediately before mine mentions, barefoot running would be great too but logistically can be quite difficult.