Booklab receives many inquiries from authors who would like to write thrillers, and I welcome them. Washington, DC has always had more than its share of insiders who actually know first-hand how things work at the Pentagon, the White House, the FBI, and the CIA. I have seen many realistic, this-could-have-happened thrillers from first-time authors in DC. Now and then Georgetown University makes an appearance as part of the plot (apparently a lot of high-level operatives are running around campus).
But often the aspiring author pours everything into one book, and then announces that she or he is "testing the waters" with it. If the book sells, then that's a green light from the industry that this is a viable avocation. If it fails to sell, well, there's always the day job. This attitude isn't reserved for thriller fans by any means. Numerous authors give me a version of the the story that they have everything riding on one manuscript, and often a beginner one at that.
This is stinkin' thinkin' (to quote Zig Ziglar) on several levels. First, it puts way too much pressure on a novice effort. Second, it assumes that acceptance or rejection by agents and editors reflects reality (if that were the case, then the oft-rejected A Confederacy of Dunces would never have seen daylight, let alone won a Pulitzer). Third, it assumes that this plot and these exact characters are the ones readers will connect to, when many veteran thriller writers, for example John D. MacDonald, had whole series of published stories with prototype characters that morphed over time into memorables such as Travis McGee. Finally, it exempts the potential thriller author from one of the most important things any newcomer to category fiction can possibly do -- plan an encore, and another, and another. By the time a manuscript is making its rounds, the author should have two or three more in the pipeline, with at least one ready to go within a few months.
For a sense of how to do this and succeed at it, you might want to consider Plot and Structure by James Scott Bell, one of the recommended books in the link at the right. He discusses thriller writing as a career. Another book I love is by Donald Maass, and Writing the Breakout Novel. Yes, they both address mechanics, but they also discuss big-picture what the life of a series fiction writer is like, and they will back up what I'm saying here. One test-market book is not an indicator of anything. Master the profession by reading everything you can, attending classes (mediabistro.com has some good ones) and writing many books. One of them just might be the one that breaks through.