I don't think an 8.5 mile ride is enough, you said so yourself you are not exhausted. WHY NOT? I am after rides.
On your bike, you will MAY be warmed up in 3 miles of riding, but usually it takes longer like 5 miles if you are not pushing it. So then you only spend 3 miles of riding in your comfort zone. Its not worth the effort. Why not ride 15 miles then youll have an actual 10 of consistent warmed up riding.
Too many people in this thread are comparing the OPs 8.5 miles with what they can do. When I started, 6.5 miles was hard, very hard. I had to rest on every hill. It took me 49 minutes to complete. The so called hills were really just blips and wouldn't even be noticed by better cyclists. Now, 8.5 miles is nothing to me. Easy, a warmup, or a speed day.
The OP said they were exerted but not exhausted and drained. I disagree that the OP should be exhausted and drained. You should feel invigorated by the daily workouts, not drained. Now the long ride might feel more like exhaustion. Intervals or hill repeats should be exhausting.
The general rule of thumb is to do a long ride of double your normal daily distance. For you, do 16- 17 miles at a slow pace. For me, I knock down the average speed by 2-3 mph and/or make sure my heart rate stays around 165 which is the upper end of the fat burn zone. Increase the total weekly distance by up to 10% a week. I prefer to do the same week twice. It can be mentally challenging to feel that I am always so pushed. Doing the same week twice makes me feel as though all my work has gone into making it easier. But do what works for you.
At first, that long ride will seem difficult. You may feel dead at the end. But keep with it. Before you realize it, that long ride will be your normal ride.