Kaunas, as Wikipedia says, is ethnically 94% Lithuanian, therefore, I can presume that Lithuanian is the predominant language in all spheres of the city life. I haven't been to the city and don't know this for sure, but you should not assume that the younger population, especially students, speaks Russian only because it's a post-USSR country. They simply have no incentive to choose it over English.
In Vilnius (63% Lithuanians, 16% Poles, 12% Russians), I had no problem using Russian in downtown stores and cafés, but even with this ethnic makeup Lithuanian was clearly predominant in the streets, and some people just didn't understand Russian.
As another example, two years ago I met a group of five math students from Tartu (Estonia, another Baltic state). Among them, only one girl spoke fluent Russian. The rest didn't speak it beyond basics.
To sum it up, I don't discourage you from learning Russian, because even in Kaunas you probably can use it with older generations and obviously for traveling, and there's a lot of useful stuff on science and engineering in it. But concentrating on it while living in an area where it isn't widely spoken, especially by your potential colleagues, doesn't seem to be the right choice.