Your sole will become tough and eventually will start hurting!
Your sole will become tough and eventually will start hurting!
The difference in the amount of deformity of various surfaces (concrete/asphalt/gravel/grass), vs the amount of deformity of the sole of shoe at contact, makes the surface inconsequential in the amount of absorption/contact force.
There are training advantages in running on the same hard surfaces every foot strike, just as there are other advantages in your feet getting used to variable landing.
If you are trying to run your fastest marathon especially, and it is on a hard surface as they should be, then you should be training your muscles to get used to that repetitive work of a flat hard surface