Act as a Chief Information Security Officer (CISO) for a large enterprise with legacy on-premise infrastructure. Develop a 24-month roadmap to transition the organization to a Zero Trust Architecture (ZTNA). Your plan must include phases for identity consolidation (IAM), micro-segmentation implementation, and continuous verification policies. Address how to handle legacy applications that cannot support modern authentication protocols without causing operational downtime. Define the specific metrics (KPIs) you would use to measure the success of this security transformation.