From the sounds of things, I'd say this is your standard friction rash/chafing and it might not be related to the diapers or the material in any way.
A few things lead me to this conclusion:
1. You said the rash is on your legs, right below the diaper, not in your diaper zone. I am assuming you mean the inner thigh? This is mostly where adults get friction rash and chafing.
2. You said it comes up when walking, friction rash is usually caused by walking, sweating and having the skin on your thighs/legs rub against your clothing to the point of irritation.
I have had friction rashes before, they suck, they come up when you have literally rubbed your skin raw from constant, repetitive exercise. I usually walk 21 K in a week, I make sure to wear light and loose-fitting clothing when I do as I have had chafing between my legs from sticking to this regiment in the past. I have also had friction rash near the top of my buttocks by working too vigorously on a rowing machine.
Diapers can be bulky and hot and if you are moving a lot when diapered, as PaddedDeist said, things can get swampy in and around your diaper area. When I wear diapers, I refrain from doing anything overly strenuous as I assume it would cause some form of chafing. When I am diapered, the most I do is craw around for a bit, lay on my back, snuggle my stuffies and play with my toys
So, yeah, this is likely friction rash/chafing caused by wearing your diapers and going for hour long walks.
The best way to treat this type of rash? If it's just slightly pink throw some zinc cream on it and it will go away in a day or so. If it's red and hot to the touch, you may want to clean the area with cool or room temp water or witch hazel, followed by you guessed it, zinc cream
If you want to prevent this from happening in the future, try going for walks wearing a lighter diaper, refrain from doing too much exercise when diapered or start using diaper cream and baby powder when taping up. I haven't had a rash (diaper rash, friction rash, etc.) in literal years and I owe it all to a nice layer of diaper cream on my buttocks and corn starch baby powder. Sometimes, before going out walking, I'll slap some baby powder between my thighs as it helps absorb sweat and keeps things fresh and chafe free
Of course, if the rash is in your diaper area or any area the diaper has touched, then as PaddedDeist has said, it may be an irritation brought on by a sensitivity to the material, but from the sounds of things this is friction rash/chafing.