For a person with a "guaranteed offer of 120k for a CS position", you seem to know very little about the position or what the reporting structure is.
With that being said, a previous poster was correct in that it will take you at least 1 year to complete training and another year to 2 years to achieve some level of technical proficiency. Furthermore, after completing these introductory years, a promotion to a 'sales rep' position certainly isn't guaranteed.
Clinical Specialists do report to a District Service Manager [or directly to the District Sales Manager in absence of a DSM]. They do have the ability to move up the clinical ladder by achieving promotions to Senior CS, Principle CS, et al.
In terms of potentially making a transition to a Sales Rep or District Manager; it is something that is possible but certainly isn't probable in the case of every CS. If you look at the just the sheer numbers, there are somewhere around 1-2 sales reps for every 2-4 CSes; and 1 DM for ever 6-10 sales reps. So just the law of numbers highlights how difficult it is to make the transition from one track to the other.
If making it to the Sales Rep or District Manager level is what you aspire to do, I certainly wouldn't dissuade you. Just know that its going to take some patience, a significant amount of hard work and touch of luck to make it.
Good luck.