The short answer is "probably as well as it could have been, in the circumstances, and the Polish armed forces actually did much better than they were asked or expected to".

When the alliance with France and the UK had been concluded, a joint plan for a German invasion of Poland had been discussed. Polish delegates had left with the message "It will take us two weeks to organise an offensive. Hold out for two weeks and we'll attack in the west, which will force the Germans to divert troops to come and fight us and you can push them back or at least hold them to a standstill while we get on with winning the war for you."

Unfortunately for Poland, France and Britain actually had their own plan for the war which went more like "blockade and encircle Germany and wait for her to collapse. There was a perception that blockade was what had won the Great War, that the Nazis enjoyed no real popular support in Germany and would be kicked out as soon as things got uncomfortable, and that it didn't really matter if Germany conquered Poland because the western powers would simply win the war and reinstate it.

So the Polish defence plan was based on a "two weeks" promise that was never a reality. Another problem was their geography; take a look at this map (actually by September Germany had also absorbed what was left of the Czech state). Germany can attack Poland from the North-West, the South-West and the North (from East Prussia, that little blob of green separate from the rest of Germany). Slovakia to the south was a German puppet and was likely to allow access if not actually join in herself (which she did). Finally, Germany and the USSR had concluded a Non-aggression Pact about a week before the invasion and it didn't take a genius to figure out that Stalin would probably want in too, given the sizable Ukrainian minority in eastern Poland.

So the Poles are literally having to plan for being attacked from all sides at once. And to make matters worse, a lot of big cities with all the industry are close to the borders so they can't simply withdraw behind the big rivers further into the country. And they felt the need to keep troops along the Russian frontier too so couldn't engage the Germans with all their forces.

Finally, Poland is simply smaller and poorer than Germany; it has fewer people (meaning fewer men of military age), less industry (so fewer factories to churn out tanks and planes) and a smaller economy to pay for all of the above. I don't have the figures to hand but Germany (never mind the Russians) had a massive manpower advantage in 1939.

There was some bad luck in timing too; the PZL P.11 fighter had been among the most advanced in Europe when demonstrated at the Paris Air Show in 1935, but just four years later it was antiquated compared with the game-changing Messerschmitt Bf109. A P.50 fighter had been designed which would have competed with the Messerschmitts but wasn't quite ready yet - one prototype had been built.

On the positive side of Poland's balance sheet - the troops were highly motivated, and in the case of the regular forces, well trained and had just had new equipment issued. A revolutionary new portable anti-tank rifle had been introduced (though it was so secret that many troops hadn't been trained on it and ammunition hadn't been distributed everywhere!). Polish pilots in particular were among the most highly-skilled in Europe even when taking into account German pilots' experience in Spain - this is a subjective judgment of course but the kill to loss ratio was something like 1:1 against Messerschmitts in much inferior equipment.

The persistent myth of the Polish cavalry charging tanks is just that - a propaganda myth put about by the Germans and swallowed whole by a Western public to whom it fitted the narrative of the "heroic, noble savage" East European. In fact, the Polish Cavalry were trained as highly-mobile infantry specialising in guerrilla hit and run tactics and anti-tank operations, and caused a certain amount of chaos in the German advance in the first few days of the war.

All in all, Poland was probably as well-prepared for war in 1939 as she could be, given all her economic and geopolitical disadvantages.

When I'm back home I'll dig out my sources, but off the top of my head For Your Freedom and Ours: The Kosciuszko Squadron by Lynn Olson and Stanley Cloud (it has another name in America I can't remember) is a great general history of the Polish experience in World War 2, focusing in particular on the men of the Warsaw Squadron in 1939, which formed the nucleus for the RAF's famous 303 Squadron in the Battle of Britain.