Another meniscus tear from running here. Mine was about 10 years ago. There is no way to rehab a tear. You either deal with it or get it fixed. It won't just fix itself. Mine turned out to be a blessing in disguise because after the surgery I was sent to rehab. In the rehab waiting room was a cycling magazine. That's what got me interested in cycling. My therapist encouraged me to start cycling (and stop running).
Like all exercise, you gotta do you. Work at your own pace....but the secret is consistency. Three 20 minute rides per week is better than one one-hour ride on Saturday. If 4mph is all you can do this week, try to bump it up to 4.5 or 5mph next week. If your body says "no", then back off. It's not a linear path. There will be good days and bad days. But the key is to keep at it. However, if that knee hurts, go see a doctor. Torn ligaments do not repair themselves and you could make it worse by trying to power through the pain. Good luck!