I am entering my 4th summer with no shoes on my horses. My 6 year old gelding and his 5 year old sister have never had shoes. The other gelding and mare both had shoes on for 6-7 months a year for 5-6 years before I pulled them.
My horses can travel ANY trail for one day. But if I try to do too many days in a row, they get ouchie. I ride long hard trails. With as nasty of rocks as anybody. A trail ride for me is easily 20 miles.
There are several factors that make a barefoot horse's feet sore.
Triming. They need to be trimmed frequently. It's like your fingernail. If they get long, you have a tendancy to get dirt packed under the nail, the weight of a horse can really pack dirt into the whiteline groove and it begins to seperate and flare the lamina. This is painful to the horse. If you keept he hooves trimmed short, You will have less problems with this. I usually trim every 2-3 weeks.
The second factor is the thickness and toughness of the sole. Do not trim the sole of a barefoot horse. Let it caulous and thicken.. Also if you can get your horse to spend some time each day on gravel, it will help toughen the sole. My horses stand in a corral that has great dirt in it and no rocks. I've refused to haul gravel in, since I compost my manure and use it in my garden and don't want to have rocks showing up in my excellent garden soil. and hence my horses just don't seem to get to that final stage where they can go day in and day out barefoot. If you will haul in some gravel and have your horses stand on it each day, It'll do wonders for their feet.
So if I go for a Saturday ride, we go barefoot. If we stay for 3-5 days and ride every day, I take some boots or other protection for their feet. I've used some of the SoleGuard by Vettec. It has stayed in my horses feet for 2 weeks, ( the company suggest it will stay 3 weeks if installed correctly) And it has provided excellent protection to my two mares. It's a little pricing and not real easy for a novice to put on the soles by themselves.
I have many friends who also go barefoot. All of the pictures below are barefoot horses. And they do great.