How must external and internal word is done by your muscles...?

To answer the problem's question, first ask "did you lift the block". Drawing the Free Body Diagram of the block, there are three forces of interest on it -- the Normal force from the ground, Gravity, and the 700N force you're exerting upon the block. Now, for the sake of expediency we'll remove the Normal force from the equation because, in all honesty... it doesn't really matter for this question.

The two forces you're really interested in are those of Gravity and yourself. Have you overcome the weight of the block? We know the block's mass, so let's calculate the block's weight.

80 kg * 9.81m/s/s = 785N.

Now, you're pulling up with 700N, while gravity is pulling down with 785N. So back to the question asked, have you overcome the force of gravity? Since your upward force is smaller than the downward force of gravity, you did not manage to lift the block at all.

Now, as far as how to find work, that is simply the vector dot product of force and distance (or the product of the magnitudes of force, distance, and the cosine of the angle between them). In the question asked, since you traveled no distance, you did no work. As far as a question of distance traveled -- force and mass can be used to calculate acceleration, but without knowing anything more (how long the force was applied for, for instance) that question cannot be answered with the above problem statement, were it not that the answer to the problem was, in fact, that you were incapable of lifting the block at all.