I haven't taken a down week since I started training seriously for a marathon September 2019. To build my base I ramped up from 75 mpw then held holding 105 for about 7 weeks then started a formal marathon program while holding around 85 mpw .
The main reason I'm thinking about doing a down week is because I haven't been getting faster since starting the formal marathon prep (about 5 weeks ago). I feel like my fitness is at a stand still and that I'm regressing (heart rate is high at the same pace comparatively to before)
Although I have no injuries, haven't became ill, I do feel like something's off and that I'm close to frying myself.
I'm not familiar with taking down weeks because to be honest, I haven't had such a successful/consistent training block where nagging injuries would force me to take a few days of very easily mileage.
So if a down week is advisable, how much of a cut down should I be going if my current marathon plan is a range of 80-100 mpw? (where 100 is the peak)