float speed = 2;
Vector3 target;
Vector3 origin;
float travelTime;
float startTime;

void SetTarget(Vector3 pos) {

    startTime = Time.time;
    origin = transform.position;
    target = pos;
    travelTime = (target-origin).magnitude / speed;

}

void Update() {

    i = (Time.time - startTime) / travelTime;
    transform.position = Vector3.Lerp( origin, target, i );

}