There is an increasingly strong demand for in-service teacher training, especially in computer science and digitalisation-related fields. However, such post-qualification programmes face significant challenges for both lecturers and participants: Heterogeneity regarding prior knowledge, differing individual timetables, workload of the primary job, and a wide geographic distribution of the participants require programmes that serve a high degree of flexibility. Since 2018, we have addressed these challenges with a blended learning, Bologna-compliant programme, which trains around 200 participants annually. Despite its high workload and comprehensive scope of content, it has a dropout rate of less than 2% and a very high overall satisfaction (94% were satisfied or very satisfied). In this paper, we introduce our programme, recount our lessons learned, and present an evaluation of 327 questionnaires former programme participants have answered. As a result, we identify 27 success factors for large-scale in-service teacher training in blended learning formats.