I define the segment, from an in-point to an out-point.
I begin rotobrushing my subject ( a man walking frame right to frame left)
I use the red rotobrush etc to refine.
AE only gives me until a couple seconds into my segment....I dont seem to be able to rotobrush the entire segment.
Im just starting AE so I'm sure theres some basic thing I'm missing
Rotobrush is NOT an easy to use or intuitive tool. You must go through the help files and do a little studying to effectively use the tool.
When using Rotobrush, at the bottom of the Layer Panel right under the time ruler there is a small and kind of hard to sere span control that you need to adjust.