What indoor exercise can I substitute for running?

I have an addiction to running. I run for usually a little over an hour each day. I would run more, but it gets dark around 5 and i get home from school at 3, so time doesn’t allow. On days when I have voice lessons or other plans, however, I sometimes don’t have time to run. What indoor exercise could I do that would actually feel like a workout? I have an elyptical, but after I run on that even for 2 hours I feel like I have done nothing.