I thought that if you put yourself into nomination for a position, Senate, Congress, Governor etc, you couldn't keep whatever previous position you had, and couldn't fall back on it if you lost? Am I wrong in that, or is that true, just not for the Presidential race?
You're wrong in that. You can keep your position. There are issues to keeping it (like not being able to perform that position as well, since your time is further divided), but Kerry was hardly the first one to do so. There have been numerous politicians in both parties that ran for President, lost, and went back to their previous position after losing.
Now if you win, you must choose which one you want to keep!