You don't have to do anything right now. You are naturally upset/angry because your husband has blown your world apart. You need time to digest, and decide what YOU want to do.
Clearly your marriage, as far as your husband is concern is less than rosy. When he says 'there's been distance between you' is that a way of saying that he's not getting enough sex? You have a young child, both work, life can be mundane, and time for each other is taken away. Often men, especially those used to having their wife's undivided attention, suddenly discover they're no longer the centre of her universe, when a baby comes along. It's then they stray.
Regardless of the reasons behind your husband having sex with another woman, he chose to do so. The woman didn't put a gun to his head and force him, he made that choice. Your husband is responsible for his own behaviour, no one else.
Don't for one minute think the OW told you, out the kindness of her heart, or because you thinks 'you deserve to know'. Nine times out of ten, it's for revenge. If this woman works with your husband or at least for the same company, then it's likely she knew your husband is married.
My advice would be to do nothing drastic right now. Most definitely gather as much paperwork as you can, and make an appointment to see a solicitor asap. There's absolutely nothing wrong, in seeing where you stand legally, whilst you mull the situation over. If you do then decide to go down the divorce path, you'll already have your advice.
If you decided to stay in the marriage, then your husband needs to go to marriage counselling. He needs to change jobs/departments, so he's no longer working with this woman. He needs to show you, that he's willing to jump through hoops, to make the marriage work. Remember, actions speak louder than words. If he gives you a good speech but he makes zero effort, then you would be better off divorcing.