I hope this does not sound "cocky." I can easily do 20 miles per day. 40 years ago 20 miles was very doable, but very uncomfortable with that heavy pack.
I am 60, and many my age (and older) can do 20 miles per day without much difficulty.
Lightweight gear has enabled me to continue to do long trips as I get older.
I am not in great shape, but in much better shape than most my age.
I am not overweight. I must have "good genes" which probably required a little luck. I think never being overweight in my life has helped keep my joints, ankles, and other body parts in decent condition, even though I have hiked and run all of my adult life, which has not caused any joint problems. Running has just been recreational. I have had many minor injuries from sports and hiking, but they heal quickly. I think that if you are not heavy, these injuries do less damage to your body. As I got older, I did have to make an effort to keep my weight stable... my food intake has declined over the years. But I do not really work hard at maintaing my weight or really think much about it. If my pants start to get a little tight, I adjust.
I get about 5-6 hours of sleep a night. This is not good, and I have been doing it for the past 20 years or so. It is starting to drain my energy level a little, so I am working on sleeping more. I drink way too much coffee everyday too.
I live at about 400 feet above sea level. 8,000 - 11,000 feet do not bother me at all, even with little or no time for acclimation. I have a yearly pass for the Palm Springs Tram. Sometimes I just hop on the tram, get off at 8,500 feet, do the 11 mile round trip to the peak (almost 11,000'), and go home. I suppose some of that is lucky genes too, and some is because I walk a lot.
I try to exercise at least 1 hour per day, 5 days a week. Sometimes I miss several weeks in a row, when life gets in the way.
I try to do a hike every weekend (day or overnights). Sometimes I miss this a few weeks in a row, when life gets in the way. Sometimes I can hike 12 or more weekends in a row. There have been a few years in the recent past, where I was able to do some sort of hike every weekend of the year.
Sometimes I run several days a week and exercise too. Runs are usually 30 minutes or less.
I eat fairly healthy food when my wife makes me. When I travel (quite frequently) I eat a lot of fast food, but always in moderation. 1/4 pounder with cheese combo meals, and Denny's ultimate omelets are favorites.
My long hikes usually involve a loop. I hate covering the same real estate twice, so often 20 or more miles a day are required to return in time.
I backpack solo, almost always. This probably enhances daily mileage or even miles per hour. Of course I have little to compare that to.
Being "older," mileage is not that important to me. Waking up above ground is. I often do shorter hikes. My wife and I enjoy day hikes together of 10-18 miles.
At times I do like to challenge myself, mileage-wise. Last summer I did a 3-day 90 mile trip in the San Bernardino's. Anything over 30 is probably too much, but I think I could do it... just not willing to try. For me it would not be fun at all.
I do not hike all day. I stop once an hour or so to drink and snack. Usually just a couple of minutes. I do take a 15 minute break each morning and afternoon -- have to look at the sights. I do not stop for lunch per se, just one of my normal hourly quick stops. I do not eat or drink while moving. In 3 season weather it usually takes 30 minutes or less to set up camp or break camp, including meals. I may not eat meals in camp. Sometimes I stop to eat breakfast or dinner on the trail instead of at the campsite. I am pretty efficient with those tasks. I spend little time collecting water. Fill the bottle(s), add chemical, and move on. Meals are boil and bag. I enjoy hiking until late in the day. I probably average 12 hours a day on the trail, including breaks. But with no one to talk to, there is no reason to stop earlier. I do not bring books or electronics, I just entertain myself on the trail all day, and then go to sleep shortly after I stop.
I am an early riser. Always on the trail by 6 or 7 am. I HATE to hike in the dark... exception is in the desert during a full moon, if it is warm. So I rarely do it.
Minor injuries usually do not stop me. I just keep going at my regular pace and the pain goes away.
Normally I hike at a fairly even pace, hour after hour.
Lightweight shoes enable me to hike a lot faster in poor terrain, compared to my old boots.
The mileage is not a big deal if you get out a lot. Even hikes in town or in local parks can make a big difference. To me, hiking just needs to be a part of your year round lifestyle. Not always easy in our busy lives, especially if you have kids. But if you have kids, just take them with you whenever possible. Family time on the trail is good, and they won't be obese.
I hope to keep up the pace for at least another 10 years, and if age slows me down after that I will keep on going until I can't.
I have been fortunate in life to be able to spend the past 40+ years hiking in the deserts and mountains of California. Life is good.
For a couple examples of 20 mile per day trips do a search in BPL for Jo Pond. On some of my trips, I write little stories for my kids, so they can remember me when I am gone. My son backpacks, so he likes them. Both BPL Jo Pond Trip reports I just copied from the story I wrote for the kids. The last Jo Pond trip with was with Craig Wisner (last year), and I was not in as good shape as normal, and had a pretty painful foot injury the first morning. But we still did some decent mileage. I will do the C2C-Jo Pond again this year (hopefully with Craig if he has time). For me it is becoming a yearly ritual.