Also k1 provides a limited time (90 days) to see if you really want to be together without any commitment. If for some reason the relationship doesn't work out then separate and no need to go through divorce process. The non us citizen must promptly leave to avoid any over stay.
Petition expires 4 months after approval. But if you have difficulty scheduling interviews and medical a note to the embassy would help. Once you have case number from the nvc you can schedule the medical and interview. Get cracking