Slavic languages are very far from English, and involve more new sounds which is difficult. The Czech course in particular isn't really good enough to learn from though. I'm struggling to follow it and I've spent most of the last year learning Polish, which is fairly similar
Very many years ago now, I did two years of (optional) Russian at school, ending up with my just scraping a GCE "O" Level pass. I found it, for sure, difficult; and basically, by the end of the two years I was fed up with it, and putting less effort into it than I might have done. Most of it is now forgotten -- I regret my overall lack of application. I've not been to Russia; but have found what I did remember of Russian, useful when travelling in other Slavic countries, as a help towards figuring out "the written word" -- public notices etc; and to some extent, for word-of-mouth communication. Particularly thus in Poland, where I've been most: Polish and Russian are quite closely related (though they use different alphabets).
Russian's big "headaches" for me (the alphabet, I found the least of my worries): Slavic languages have few words with any similarity to their equivalents in English, or in other western European languages. Russian words are highly inflected -- numerous and assorted variations in their endings, according to their role in the sentence. Russian "does" verbs in an insanely complicated way: two forms of every verb, depending on what job the word is doing.