Totally not judging. I was a new runner once too. Running is very specific activity, were the majority of improvements come simply from just running more. However, running more is a very slow process, you can’t go from 0 miles, to 10 miles, and expect them to be fast. While you are fit, you aren’t running fit, it’s like being fit and not being lift heavy fit. Your plan is a get to the finish plan, which is good for ramping and getting to the finish line, but isn’t really going to build speed, but that will come if you continue to work at it. Don’t get discouraged, the improvement when you start are often the the biggest jumps you have. Good luck