The cat is out of the bag now, right? Once you cross that line it is hard to walk it back. For men, it is easier to carry on a physical relationship without getting emotionally invested, and likely if you try to take back the latter, he will protest.
For women, however, as is part of their genetic make-up, the physical connection solidifies the emotional one, so it will be more difficult for you to keep it casual as you are being physical too.
Reducing the number of times you see him a week might help, and even stating you are not exclusive and both free to date others, could also help, but I predict you are already emotionally invested and would not like the idea of him sleeping with others.
In reality, and I know this is not helpful, you should never have allowed it to progress past friendship. You were raw and wounded from your marriage breaking up, but this is the time to re-evaluate how you deal with your relationships because what you are describing in your marriage and this FWB relationship are at the extremes.
You have been in a marriage that by your own admission was one in which you were "not close for years" and yet, less than 6 months later you have jumped into a new relationship. It appears that you are a "reactive" person . . . not analyzing much what is going on in your life, and waiting for things to happen to you . . . your husband to end a bad marriage . . . a friend to hit on you, presumably.
This should be a wake up call to start identifying issues early or at least timely, and making a plan for action that you follow through with. So, in a bad marriage, one in which you were not close for years, did you attempt to change that? Did you both go to counseling? At what point, because you were surprised by your husband's actions to end it, had you accepted that this was the marriage you were to have? And when the friend became more than a friend, in that moment, what were your thoughts? In that crucial moment in which you could decide to allow it or not, why did you not put the breaks on it then? Why, only now, are you analyzing things and trying to act?