The Finns indeed did fight very galantly and utilised their limited resources very effectively. When all that is said, it must however also be added, that they were very much aided by Soviet incompetence. By 1939 Soviet execution of military leadership was extremely crude. For instance artillery support could best be described as massing huge numbers of guns and then firing arbitrarily in the general direction of the enemy. That gave a nimble army not afraid of seizing the initiative like the Finnish army great opportunities. By early 1940 the Soviets had already learned a lot however, and the Finns had no chance of withstanding the final massive assault on the Mannerheim Line.
The Germans by 1939, although not on 1940 standards (actually Blitzkrieg had not been invented yet), were way ahead of the Soviets, and could not be expected to execute the same blunders. OTOH the Germans had to be very bold, as they needed the campaign decisively ended in a few weeks.
With a more complete Polish mobilisation and a more in depth deployment I would not exclude that one or more key German formations could be defeated and the German offensive bogging down. By 1939 the Panzer Divisions still did not operate in semi-independent Panzer Korps (a key element in Blitzkrieg) and a more co-herrent Polish defence thus could cause serious problems for the Germans.
In an attritional war the Poles alone will not stand a chance, but apart from perhaps the French launching an offensive at the sight of German trouble, I also wonder how the Germans will react to this. Well into the 1940 campaign Hitler tended to panic at signs of trouble and the German leadership in general apparently were shocked when France and UK declared war - and utterly un-prepared. For instance the artillery ammo stocks were depleted after the OTL Polish campaign, in a drawn out campiagn that would force the German army to dig in and leave what is left of arty ammo for defensive operations.
If the Poles have prepared big stocks of ammo and supplies in central positions that could provide an interesting situation (Poles taking Berlin?!), but more likley they also dig in and wait for news.
But anyway I simply don't think Hitler could survive anything but a quick and decisive victory in Poland. Nobody would be paralysed by his political or military successes, on the contrary any goose stepping Feldwebel could easily be brought to see how disastrous that man had been to Germany - having bluffed and failed...
Regards
Steffen Redbeard