I have started walking for exercise every day. Each week I add ten more minutes to the time I am on the pavement. Starting at 40 minutes, I am now up to 100 minutes per day. Is there a point at which additional time will be detrimental to me physically - say harm to knees, hips, feet or some other body part? Or is more always better, and it will just become a matter of running out of will or time?
You’re kind of asking “how long is a string?” No, there is no hard and fast rule for every person–my limit is less than yours because I have a bad knee. Before my knee went sideways I could hike miles with serious vertical elevation change and have no problem but now I’m older and my knee is dodgy so my limit is much less. Basically, if it starts to hurt then back off a bit.
More exercise isn’t always better. Pushing too far can lead to decreased performance, general fatigue, and problems with appetite, sleeping, and mood. Over exertion is counterproductive. That is aside from the potential of actual injury, which is always a risk even when you’re exercising in moderation (accidents happen after all).
Listen to your body.
I would walk 25-30 miles every day when I was young and mobile and paid very little to deliver junkmail house-to-house around hilly San Francisco bay area terrain. Knees still worked then. Even into my 50s, a 15-20 mile day afoot was no big thing. That was before necessary medical work on knees from which I’m still recovering. But I’m working on redemption. The post office is 4.5 miles and 1000 feet below us. I’ll feel restored when I can stomp the round trip.
Another exercise routine, recumbent biking, will take longer because MsRico won’t have full use of her wrist for a year. We have semi-recumbent BikeE AT cruisers, MUCH more comfortable than diamond-frame bikes - like riding choppers where we’re the engines - but they require strong arms. I still prefer biking to walking; more chances to sit down. 
My rule for any ambulatory exercise: start out uphill, return home downhill.
If OP or anyone questions their abilities, consult a physical therapist.
US Government guidelines recommend a lower limit of 150 minutes a week of moderate intensity exercise, but they don’t currently have an upper limit. A handful of studies have come out in the last few years ago that suggest that too much exercise may be bad for heart health, but still not as bad as being sedentary.
You can read one of the studies here. One of their figures compares all-cause mortality to the weekly amount of “physically strenuous and sweat-inducing activity” such as “cycling, speedy hiking, gardening, [or] sport.” One caveat: They measured all-cause mortality and non-fatal cardiac events, but didn’t try to measure things like (skeletal) muscle injury and joint damage. The authors found that anything between about 8 and 14 hours of such activity per week is associated with quite low mortality, but the sweet spot comes at about 11 hours per week. That happens to be just about the amount of walking the OP reports doing, but he doesn’t say how strenuous his walking is.
I’ll be 67 next month. My Border Collie and I walk over 12 miles (three 4 mile hikes) a day over dirt trails where we live here in southern California. Some parts of the trail are quite steep (great for elevating one’s heart rate). I don’t have any health problems from this walking. My resting heart rate is in the high 40’s.
Walking is great exercise, but like everyone says, “See your Doctor before beginning any exercise program”.
[Moderating]
Since this is medical advice, let’s move it to IMHO.
My sister and her husband walked the Pacific Crest Trail in their mid-sixties.
There is no upper limit as long as you don’t have some physical condition compromising your abilities that can’t be cured by walking more. Humans are very, very good at walking.
Not sure about walking, but the belief that “too much jogging will ruin your knees” is pretty much a myth. I don’t have time to dig it up, but some studies have shown that runners’ knees tend to be healthier & stronger compared to non-runners. I’m guessing there’s a similar correlation with walkers.
The biggest risk with lots of walking or running is falling or getting hit by a vehicle, not bodily damage due to over-stress. There’s also the risk of long-term damage due environmental factors (skin cancer from UV in sunlight, frostbite, etc.).
And as someone as already mentioned, listen to your body… if you’re experiencing pain, stop the activity and treat it before resuming.
I do about between 1.75 to 2 hours of miles a day. Physically I could do more, but I think mentally this is about how much I can handle day in and day out. (The distance varies because sometimes I take the short cut home and sometimes I go the long way).
I think you will know when you’ve hit your limit. Pain is a great indicator. So is fatigue. Back when my commuting distance was a bit longer than it currently is, I remember how I would drag during that last mile in the evening, especially during the summer. So I think as far as a daily walking regime goes, 7 miles is my preferred upper limit.
Yeah, when I first saw the title, and then when I started reading, I kept thinking “No, no, no. Don’t ask us, ask a physical therapist!” I’m amazed how much more function I have thanks to a couple of visits (and actually doing the stretches/exercises they gave me).
I’m 65, I have walked outside for at least an hour a day, and usually more, for the past 35 years. I am in great shape, still working and taking care of myself.
I tell people “If you walk outside every day for a month, you will do it for the rest of your life.”
Keep on walking!
Humans evolved for thousands of years to walk long distances for hunting and gathering. We are built to walk, and I’ve walked/hiked between 45 and 120 minutes a day with my dog for quite a while and I’ve seen no ill effects on my joints. Compared to jogging and running, walking is a relatively low impact exercise. Not as low impact as swimming, but still fairly low impact, unless you are speed walking. I even have a treadmill for when the weather doesn’t cooperate. As has been said everybody is different, so you should discuss this with your PCP at your next regular check-up and see if he/she sees any potential risks with increased walking. If you live in a northern climate as I do walking outside in the winter can be problematic when the trails are icy, but there are things you can do to mitigate the situation. If I were you I would gradually increase your until your body starts complaining. There are free apps you can download that will count your steps. Your phone may do this by default. Set a goal and try to meet it daily. Once that gets too easy you can increase your goal. I’m at 15K steps a day right now and so far so good. YMMV.
Even if the rest of your life is twenty minutes? That’s walking in February in Wisconsin.
-20º F wind chill yesterday, and a walk meant climbing chest-high frozen snow drifts, then sliding down the other side into traffic just to cross the street… Limped home, thawed my fingers and toes in the shower and walked in place for an hour.
ETA: Okay, I wanted sympathy, but I’ll admit it’s 30º higher today. Took a shorter-but-brisk walk. The thing I love about walking is that you don’t need to spend money on equipment… you were born with it!
I think it’s safe to say that no one should walk more than 1440 minutes per day.
I walk a lot because I can’t drive. I agree with the general consensus: listen to your body, and if you have medical issues or previous injuries, ask a doc or PT.
I just wanted to add that footwear is important. A sneaker that has more flex in the midsole is better for your legs and feet when walking long distances. I have high arches, so I need more cushioning and feel better if there’s more arch support. Consider your own areas of concern–Knees? Achilles tendon? Hips?–and do a little research to find out what features to look for. Also, replace shoes every 1,000 miles or so.
That could be surpassed by crossing the International Date Line with careful timing and Jesus Feet.
Does spending eight hours a day, six days a week, on my feet walking back and forth at my station on the assembly line count? (Seriously, does standing count as exercise? Turning and moving a part from one table to the next?)
That study may be susceptible to the argument that correlation is not causation. I had a solid argument to still run until I was 45. That was my last physical fitness test before I retired from the Army. I needed to be able to run 2 miles for time in order to pass. If I had a demonstrable physical issue I could have had a separate event in lieu of the run. I didn’t, yet. It just hurt.
I do have a family history that included both lots of arthritis and a knee replacement without running being involved. I am also quite a bit taller than average which sees increased skeletal issues. Most importantly by the time I was in my late 30s my knees and hips would ache the next day after a 2.5+ mile run in a way they didn’t when I was young and running all the time. By the time I was in my 40s I really needed a day off between runs for my knees and hips to recover. Guess what I don’t do as exercise after retirement? I am no longer a runner because it hurts to run often.
If the study didn’t at least try to control for people like me who stopped because of knee issues it may not show what you think it shows. It may just show that people who have stronger and healthier knees are more likely to stick with running as exercise.