Duration, intensity or both?
For either, adequate recovery and energy replenishment is needed.
"Can running, walking, etc. be harmful in the long term if you exercise too much?"
Regular physical activity can help in reducing the incidence of KOA [knee osteoarthritis] and its economic burden, in part because of its beneficial effects on weight control [3, 4]. In addition, physical activity and exercise have been widely recognized as essential components of clinical management of people with KOA [5, 6].
What are the perceptions about running and knee joint health among the public and healthcare practitioners in Canada?
Running causes joint stress. Any mechanical or biologically mechanical part will not function well if excessively stressed without enough recovery. Here are some studies to sift through: https://pubmed.ncbi.nlm.nih.gov/?term=running+knee&filter=pubt.systematicreview&size=100
Here is one study outside of that search:
Conclusion: The injury incidence proportions by anatomic location between ultramarathoners and non-ultramarathoners were not significantly different (p = 0.798). The pathologies with the highest incidence proportion of injuries were anterior compartment tendinopathy (19.4%), patellofemoral pain syndrome (15.8%), and Achilles tendinopathy (13.7%). The interpretation of epidemiological data in RRMIs is limited due to several methodological issues encountered.
A systematic review of running-related musculoskeletal injuries in runners