My family are Kuban Cossacks and we have Circassian bloodline trough my grandmother's ancestors. It was not so rare among us. But most Kuban Cossacks are pure Slavs. Genetically I'm closest to Ukrainians.
Don Cossacks are similar to Central Russian populations.
Languages may change but genetics shows ral origins of people.
They can not possibly be much different from the neighbors in Baltic states, Eastern Ukraine, Belarus.
Further towards east are other core Russian regions, like Ryazan, Tambov, Penza etc., which account for probably another 15 - 20%. And than you have Siberia and Far East that was mostly populated by migrants from central regions. North Russia is rather sparsely populated and accounts for maybe 5% tops. All this talk about Russians being so diverse and different from other East Slavic countries is nonsense.
BTW, I have created a thread before about those disgusting north Russians, that, for some reason, a lot of Russians from various anthroforums feel ashamed to associate with: