You do have a nice entropy calculation but you did make an error. That is because basically, private keys are case sensitive, meaning the total number of combinations or permutations rather since "3A" and "3a" or "A3" and "a3" are not the same should be something like:

(A-Z)×2 = 52 ; this is because, number of upper case +  number of lowercase has to be taken into account.
Also the 10 digits (0-9) so in total we have 62 permutations.
To put into simple permutative perspective, we have 62²⁰.

As for the OP you can't run such a brute for because it's like wasted power since the computer will end up running for years. Only the concept of quantum computing may pull it off but it's still not applicable here.