Shaun, this recurring question from my students, "How much are Russian and Ukrainian different?", makes me think about writing a post on that :) The languages are different, much more different than Polish and Ukrainian, for example. But what is more important is the language situation in Ukraine now. More and more people, companies and mass media start using Ukrainian, and so you do need to speak Ukrainian if you want to visit Ukraine, BUT still, there is a big "but": it is very good to be able to understand Russian. As far as you may suddenly hear Russian in the flow of Ukrainian, sometimes you will hear, e.g. radio interviews where a narrator asks a question in Ukrainian and the person answers in Russian without translation. If you know Russian, it may help you to understand Ukrainian (about 60%), but knowing Russian will not help you speak Ukrainian (=produce your own texts) because words, pronunciation and grammar (to some extent) are different. See this: https://www.pinterest.com/pin/714242822131262088/ (the numbers show how much there is in common between certain languages).
To conclude, I would say that the key to other Slavic languages is not Russian, it is Ukrainian, as other Slavic languages share more in common with Ukrainian than with Russian (ancient roots, grammar, etc.). But people tend to start with Russian, because there are more Russian speakers, and Russian is understood on the grounds of the former USSR.