No particular series is needed. Just get back into training, from a little lower level than where you left off, and do a few cardio sessions per week, and 2 or 3 weights sessions a week and start building your strength back up.
The whole point about the STS system is that as long as you are already in some degree of regular fitness program, you can jump right in from wherever you are at. It doesn't matter what point of relative strength you begin from, you will gradually make progress through the three cycles: how could you not grow with a well though out progressive program?
You don't have to do anything to pre-qualify you to do the STS workouts: i.e. you don't have to lift what Cathe does. You always lift what is right for you and progress from there.
So, in the end there is no series that will best prepare you for doing the STS because the STS is the best system for enabling you to do the STS. You get stronger by doing it, just like you learn to speak a foreign language by speaking it. That's all there is to it.
And once you have completed it, you start again, only this time from a stronger beginning point, so you make even greater gains.
Clare