I had surgery to repair a torn meniscus about a year ago, and my knee still hurts if I am active for too long. Shouldn't it be healed by now? Or is there another problem?

May just be arthriti. Its common to have articular cartilage wear in the knee called osteoarthritis. Meniscus pain is sharp & acute, common with twisting and squatting. Arthritis pain is usually a dull achey pain with first getting up, after long periods of activity, at night at the end of along day, with stair climbing, rising and sitting, getting on and off the toilet. Xray shows space lose. See you doc try injection.
Either. It may be healed and you are feeling scar tissue. It may be a new injury. Remember, surgery tries to correct an anatomical problem, but it cannot guarantee relief of pain; especially if there was damage down that could not be repaired.