We can account for the energy lost to friction for a sliding object as...

would it be friction force/time traveled?

It is the friction force times the distance travelled. This is the work done against the friction force, which is work transferred to heat.

If you look at the sum of kinetic and potential energy, then this quantity is not conserved, because of friction. Without friction, the object would gain exactly as much kinetic energy as the loss in potential energy.

Due to friction, part of the kinetic energy is lost as heat. This is then accounted for by the internal energy of the object and its enviroment.

Internal energy is the total energy of an object at rest (due to the motion and potential energies of the atoms that make up the object). At higher temperature the internal energy is higher. If heat is transferred to an object, the internal energy rises and thus also the temperature.