Simple linear interpolation function at a constant speed.

Lerp.cs

public float driverFloat;
public float currentFloat;

private float Interp(float current, float target, float divisor) {
    if (target != current) {
        return current + (target - current)/divisor;
    } else {
        return target;
    }
}

void Update() {
    if (isHandAttached) {
        driverFloat  = gameObject.GetComponent<LinearDrive>().linearMapping.value;
        currentFloat = gameObject.GetComponent<LinearDrive>().linearMapping.value;
    } else {
        currentFloat = Interp(currentFloat, driverFloat, 10f);
        gameObject.GetComponent<LinearDrive>().linearMapping.value = currentFloat;
    }
}