Even with the best case scenario for Hitler it would take him at least a year to defeat the country.
Why we didn’t fight?
Because we were readying for Hitler from 1933. As he built up alliances the strategy changed for building fortress Czechoslovakia that would be too expensive to defeat for anyone, and the complete military built up with three rings of inner defensive fortresses and the world’s most modern army were to be ready by 1942. Obviously, it wasn’t complete by 1938, but it was more than ready to fight Germany alone.
The main strategy that was developed together with France was that in case France was attacked, Czechoslovak army would immediately open front against Germany on its side. Should Czechoslovakia be attacked, France would do the same. Germany could would not be able to deal with that in 1938, not in 1939, not in 1940.
So clearly, reason number one was French - as well as British - complicity in encouraging Hitler to occupy Czechoslovakia. With France leaving its commitment, Czechoslovakia could hope for long defensive war with best outcome on par with what Finns got in their war with Russia, but with no chance in victory.
However, the French and British encouraged not only Germany, they also made it clear to Poland and Hungary that they can take what they want. That was important especially in case of Poland which would not dare to endanger its alliance with France and UK. Look at the map. Not only the numbers were much worse (although Polish and Hungarian armies were desperately badly equipped compared to Czech), but especially geography made it completely impossible to fight the three countries.
The third issue was the existence of German minority that was fiercely pro-Nazi. By the time of Munich Agreement, we have already lost some 100 soldiers in fights with German paramilitary units within the own territory, and when the full mobilization was issued, some 50% of Germans didn’t follow.
The reason is that we had a fighting chance to do our own war if left alone to deal with Germans alone. But not in situation when France and UK encouraged also Poland and Hungary to join the party.
http://pevnosti.kvalitne.cz/files/ovc-8.jpg
FYI, this is one of Czech border bunkers. Germans tested their best anti-bunker canons after our surrender, firing from mere 50 metres (in wartime situation, they could never get so close).
The test resulted in utter failure, they could not take our bunkers in 1938 and they could not go around them like they did in France. Hell, they could not penetrate them even with newer equipment in 1942, the year Germany finally did have THE army you were talking about.