Unproductive Runs

Hi! I am currently about halfway (5 weeks) into a half marathon training program. My mileage is increasing a good amount each week, but I feel like my fitness is not improving. It doesn’t feel like the runs are getting any easier and I’m not getting any faster. My garmin watch says that my fitness is actually decreasing. What should I be doing differently? I feel like I take my easy runs pretty slow, but I have a really hard time hitting the paces on my fast days, which is why I feel like I am not improving at all. Any advice?