In general, Calluses occur due to friction caused by incorrect running posture or technique, humidity or heat.

If your feet don't land on the ground properly, the friction created will create calluses. Therefore, the location of the calluses can be an indicator for finding posture problems.

Calluses on toes
It shows that you are “pushing off” or stepping foot too hard on the ground to move forward. The feet should be lifted and not pushed forward.
Solution: focus on lifting up your leg when you run. Some runners imagine a rope tied to their knees and the rope being pulled up with each step

Calluses on the front or side of the forefoot
Calluses at this location are usually caused by feet landing accompanied by a large force. This is usually caused by over-striding or the foot landing not snugly in the center of gravity. Solution: To make sure your feet land on the center of gravity, slow down or shorten your stride. When you have the correct posture, the pace can be increased.

Calluses on the heels
These calluses occur due to heel-strike or landing on the heel before the middle or forefoot.
Solution: Focus on landing on the forefoot. And again, slow down or shorten your stride to crack this problem.