I work at a place that uses an extensive amount of JCL, COBOL, REX, MK4, VSAM, DB2, and more, and I would say that learning the language is only a small portion

I work at a place that uses an extensive amount of JCL, COBOL, REX, MK4, VSAM, DB2, and more, and I would say that learning the language is only a small portion of learning the system, batch processing and the database. Learning COBOL without learning how the mainframe works in its entirety is leaving out the major portion of what the language is used for. It would be like using a C64 your whole life and then learning Java from a book, but not having a system to apply it to. The leap is just too great without immersing yourself in ISPF and all the associated proprietary software your shop.