Well, back in January I was able to reach 248 in a Ramp Test. I’m 67 weighing 65 kg/143 lbs with Strava showing 5400 miles last year. That Ramp Test came after Sweet Spot Base mid volume II. Your milage may vary; my background includes a lot of distance running when I was younger, but also a couple interim decades with almost no endurance exercise.
Make sure your heart and cardiovascular system are healthy enough for this kind of work. My last stress test was a couple years ago; it’s time to go again.
I’ll also add that I found the Sustained Power Build mid volume plan to be too hard, and my testing spiralled downward with me recently wimping out to get a 233. My previous 2 years with the Build plans gave similar results. Joe Friel’s “Fast after 50” says we need intense workouts, so I continue to use TrainerRoad to provide these.
I agree that 300 watts may not be very realistic for us, but I’ll continue to explore what IS possible.
Did I mention that you should have your heart checked?