How about some cross training to reduce the impact on your joints ? You don't have to give up running. You could swap a couple of days with cycling. Do you have a bike ? If so, then how about riding it twice a week instead of running ? If you don't own a bike, do you have access to a pool ? Swimming is another great low impact activity you can do as cross training.
How about walking ? I love walking. I walk everywhere. it's wonderful exercise. take a long walk a couple of times a week. Do you have a gym membership ? If so, try spinning. that's a high intensity low impact cardiovascular workout. it's basically an indoor cycling class. it's lots of fun and a great workout. gentle on the joints too.
You don't have to do high impact exercises to get a good workout. Low impact exercises like Zumba or dancing are good too. And you can do plank without being on your wrists. Do the modified plank on your forearms or on a stability ball. If you really want a challenging exercise, try holding plank on a stability ball.
And you might want to ask your doctor if you can do yoga. I have bad knees and I've found that yoga has helped make small improvements.