In my opinion its not the miles that count, but the effort you make. On the week you aren´t doing many miles, an then you do that steep hill ride. To do anaerobic wor you first got to build muscle with more fibers, and more coronary arteries in aerobic work. I thing yore not riding enough on the flat as to gain more air, and the hill ride is just burning your legs but not improving your endurance. You should check the training sessions has some cool advice on resistance and power. id improve resistance at first and then power.
I might seem like I have bought stock on Garmin or Polar, as lately i always recommend this, but you should get a heart rate monitor and check on what zone you´re pedaling. Miles dont count, speed doesnt count. What counts is your heartbeat and what heart zone you are working on. The heart rate monitor is the RPM of your engine. If you ask me if 70mph is forcing the engine, then id ask you what are the RPM. If youre drving a Gordini at 7000rmp id say its too much. If youre on a Lambo at 2000 id say its ok.
The more resistance and power you gain, the lower your bpm will be, so blood flows correctly to muscles and you dont get burnt