You need to normalize and also take the position of the body itself into account. The following code is untested, but should work.
@Override
public boolean touchDown(int screenX, int screenY, int pointer, int button) {
camera.unproject(touchPosition.set(screenX, screenY, 0));
// calculte the normalized direction from the body to the touch position
Vector2 direction = new Vector2(touchPosition.x, touchPosition.y);
direction.sub(myBody.getPosition());
direction.nor();
float speed = 10;
myBody.setLinearVelocity(direction.scl(speed));
return true;
}