There is nothing wrong with doing a week 8 run in 3.8km. That is perfect. The goal is to be able to run non-stop for 28 minutes. Distance is irrelevant. Your pace is fine. Keep your head up and keep running, your doing really well. Your almost at the finish line. In week 9 you will be running for 30 minutes. Forget about 5k. When you graduate you could start running towards completing 5k by doing Parkruns or entering a 5K fun run.
You might want to try running out doors for a change too - much easier to adjust your pace - you actually feel if you're getting somewhere - and I just hate treadmills - so dull! (but that's just me!) - and that other stuff about running 28 minutes and not sweating the distance is easier too!
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