Internal carotid artery dissection and exercise...my dissection has mostly healed (2mm aneurysm remains) over the last 20 months. what kind of exercise can i do and at what intensity?

Not enough info: What was the cause of your carotid disease? Do you have generalized atherosclerosis? Was your dissection idiopathic? Do you have arteritis? How was it treated? Have you had a cardiac stress test since heart disease is strongly correlated with atherosclerotic carotid disease. Your question cannot be answered in a vacuum.

Ask UR cardiologist: Whoever is following you medically needs to be involved in this discussion, as their recommendations is what you'll need to follow closely. Give your doctor a call, and she what s/he says. You don't want to aggrevate the remaining 2 mm aneursym present. This is a great question, and glad you are exercising carefully!