Uninomicon

This function does only respond with the current remaining distance if the path is straight. It will thus return null or infinite instead of the correct distance. There is a way to calculate the real distance by combining all calculated paths but keep in mind that this is performance heavy.

Example On stack overflow:

public static class ExtensionMethods
{
    public static float GetPathRemainingDistance(this NavMeshAgent navMeshAgent)
    {
        if (navMeshAgent.pathPending ||
            navMeshAgent.pathStatus == NavMeshPathStatus.PathInvalid ||
            navMeshAgent.path.corners.Length == 0)
            return -1f;
 
        float distance = 0.0f;
        for (int i = 0; i < navMeshAgent.path.corners.Length - 1; ++i)
        {
            distance += Vector3.Distance(navMeshAgent.path.corners[i], navMeshAgent.path.corners[i + 1]);
        }
 
        return distance;
    }
}