Hi, I've started running 5.5km everyday for a week now. On the first day, it took me 38 mins to finish the run. But gradually, I keep getting slower and slower. On the 5th day it took me 48mins to finish and the result afterwards is inconsistent.

Asking the people for tips to improve my running time. Should I add rest between days? Supplements? Or anything really, my ultimate goal is just to hit 5km under 20mins.

Note that I'm a 23 year old male currently weighing over 98kg, so I'm a bit slow.