Converting legacy automation tests to Specflow....or not?

Now that BDD is being fully embraced at the company where I work it’s flagged up a couple of question in my mind. We’re writing tests in a gherkin syntax in Visual Studio using Specflow, and writing the c# code to allow us to run these tests in automated fashion whenever we need to in the future
My questions are:

  1. Would these tests that we have written in specflow, be executed ad infinitum from this point on? My concern would be that eventually this suite of tests would build up and we’d have a huge rack of automated tests that would need to run, taking a good while to run.
  2. We also have a separate regression pack, which whilst automated and written using POM, isn’t written in Specflow as it was set up before Specflow was introduced. As a result, it means we have automated tests written in specflow(point 1), and a regression pack not written in specflow. Is it an issue to have a structure set up like this? Or should we be looking to migrate our existing regression pack over to specflow scenarios