I've read some of the same about the English longbow. 100, even as high as 150. Effective range for military uses at the time, up to 400 yards. I imagine raining down arrows on formations of enemy. Wild stuff.
Howard Hill shot a very heavy longbow even when he was older. I know he was shooting 150lbs when he was at his peak. I remember him saying it was so he would have a flatter trajectory on his trick shots.
You will likely be fine with 50 lbs. You will increase strength quickly. The thing to watch out for is getting sloppy form and bad habits while you are gaining strength in those specific muscles. Try to learn exactly what good form is and limit your practice to as many as you can shoot properly,not as many as you can shoot.
