What are the private and public keys associated with an Apple Distribution Certificate?
The certificate contains a copy of the public key. Having the certificate without the private key is meaningless in terms of distributing iOS programs.
The private key is created automatically when you create the certificate signing request as described by the linked to guide. The private key is stored on your local computer, and can be found in the Keychain program.
The private key is not contained in the certificate nor it is not downloadable from Apple's developer web site. You will want to keep the private key, well, private - and secure.