Like most software (including MindManager itself), you don't need to know all of it to get benefits from using it. Reading the whole manual is not an essential step - the section on using Express Mode will get you started without knowing everthing. The manual is also unlike most software manuals in that it relates to practical usage rather than just a list of features and functions, so appears bigger for this reason.
The biggest change is to move from using MindManager to make to-do lists to using it to create plans, and letting ResultsManager do the list-making.