A question for the culinary types.

I recently made a meal (chicken vesuvio, though not quite the version described there) that turned out pretty well, except the texture of the chicken breasts was less than ideal. The outer portion was great, but the core was kind of tough, and I’m wondering what the reason for that is.

The recipe calls for the breasts to be lightly browned and then put into the pan with potatoes, broth, and cooking wine and simmered for about 12-18 minutes. My impression is that the browning part went great (which is why the exterior of the meat was in good shape), but the simmering is where things went wrong. Could it be that the meat simmered too fast, or reached too hot a temperature? I’m supposed to get it up to 160 degrees; after 12 minutes it had already shot past that. My stove tends to run hot, so I feel like maybe it would turn out better if I reduced the heat (it calls for medium-low, so I could go to low) and let it cook a bit more slowly. But I don’t actually know the dynamics of how these things work, so I could use either confirmation of my theory, or an explanation of what’s more likely to have been the problem.