Is this really a valid concern in Russia?
Yes, of course it is. I am very worried by the number of people who cannot see that it must be - a large and heavily armed force whose primary declared purpose is to defend against YOU, and who has pointed large numbers of weapons at YOU in pursuit of that purpose is always going to be a major and valid cause for concern. Even the weakest strategists have heard the phrase 'attack is the best form of defence'.
And that cuts both ways. If Russia feels threatened by NATO - and it would be very strange if they did NOT - then that might lead the Russians to decide that their best strategy is to attack NATO; Not due to any particular animosity, or in retaliation to any particular incident, but as a defensive move.
It is therefore of vital importance that NATO makes it very clear, by word and (more importantly) deed, that invading Russia is not something they intend to do. Because assuming that the Russians agree that such an idea is ridiculous, just because it looks ridiculous from the perspective of NATO HQ in Brussels, would be massively dangerous.
If NATO were planning to invade Russia, then they would obviously have a lot to gain by claiming and trying to pretend that such an idea is crazy and impossible. So the mere appearance that NATO thinks it is crazy and impossible is no comfort at all to Russia or Russians.