Despite Polish propaganda, Polish commanders knew well they were not able to beat Germany by themselves. Polish plan was to hold their lines until France would start offensive in the west. To achieve that Poles needed pretty much all the forces they could mobilize. According to Plan Z (Polish defensice plan against Germany) all Polish regular units were to be used against Germany. Polish eastern border was to be protected by KOP (Korpus Ochrony Pogranicza, Border Corps) and some reserve units. Enough to keep quiet Ukrainians and Belorussians (who had no love for Poland), not enough to stop Red Army (as it happened IOTL). In case of joint German-Soviet aggression Poland had no chance whatsoever for any successful resistance. Polish Army was simply too weak.
Poles knew that very well, so the main goal of Polish pre-war diplomacy was to keep an equal distance from Germany and USSR : an alliance with one of those powers, which both were rather hostile towards Poland, might be seen as a provocation by the other. Another goal was to keep both those powers as separated as possible. Unfortunately it failed.
One interesting fact is, that for quite a long time Poland didn't actually have any serious defensive plan against German agression. Until 1933 Germany was demilitarized and Reichswehr was too small to pose any serious threat to Poland. Polish commanders were much more worried with the possibility of Soviet invasion and that was the reason Poland mantained big army.
When Hitler took over in Germany and started its remilitarization, Poles recognized the Wehrmacht might become a threat in a future. However, they couldn't have forsee how quickly it would happen. They didn't anticipated the Nazis would have been willing to almost kill their own economy to prepare for quick war. Poland had big plans for modernization of Polish Army, but it was to be started in 1940s, after successfull modernization and development of Polish heavy industry and economy. BTW, Hitler's generals also anticipated the start of the war in 1940s. Therefore, Polish defensive plan against Germany was created hastily and had some serious flaws.