I cannot sit on the toilet long before my legs go to sleep - why?

This is a common complaint and you can rest assured that it is not a serious problem. Essentially what is happening is that the position while seated on the toilet is allowing some of the nerves in your legs to be compressed by your body weight, which causes numbness and tingling on your legs in the areas where the nerves run. This is exactly like what happens if you accidentally fall asleep on your arm and wake up with pins and needles in the fingers. Also, just like with that problem, changing position will relieve the symptoms and no harm is done. You should of course report to your doctor any concerning symptoms, such as numbness or tingling in your lengths in other positions, weakness of the muscles in your legs, or any pain or strange symptoms while walking or performing other activities. However, it doesn't sound like you have any of those symptoms. As to why you are experiencing this now but did not previously when you were younger, I am not sure. It is possible that the extra weight you have put on over the years is the culprit. Regardless I would not be overly concerned about the problem.