Both the World Health Organization and the UK National Health Service provide guidelines for physical activity, more specifically a lower limit for moderate and vigorous activity.
However, I could not find what the scientific consensus is regarding an upper limit. Can running, walking, etc. be harmful in the long term if you exercise too much?
Note: European male student in his early twenties (183cm/77kg)
Impossible to know.
We are not copy and pasted, if you have brothers and sisters you might notice that even though you are genetically almost identical you still look completely different simply because the smallest dna difference can make the most absurd disparity.
But I can tell you some factors to study your own upper limit.
Your overtraining limit is based on :
Your DNA
Your mothers hormonal balance and diet while you were still a fetus inside of her
How much your mother was rubbing and cuddling her belly while you were a fetus inside of her, this activity influences the feel good chemicals in her brain which in turn influence your brain and body development.
How active you were growing up
Your hormonal balance, which itself can go up and down drastically through the day and even more through your life. Something as simple as talking with someone you find attractive can send your hormones to the moon for a few hours.
Your diet, and everyone tends to react similarly to food but we all have some personal differences in how our bodies react. For example without glucose everyone dies, but without fiber some people live a hellish toilet life while some are fine.
Your training experience. For a world record holder 6000 pull ups in a few hours is not overtraining, for a beginner it's certain kidney failure and almost certain death. Your organs need to adapt too, not just your muscles.
How warm your muscles are, warming up is not just a figure of speech, muscles perform better at higher body temperatures...with a limit ofcourse. Too cold and you underperform, get tired sooner.
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].
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
