I was a moderately successful female runner (10:30 for 3000m, sub 18:30 for 5k, etc). My problems stopping me running faster were due to being held up by biology, due to being female and my body developing accordingly. Before I hit puberty, I didn't have breasts that wobbled and were extra needless weight (I actually had them reduced surgically), or periods (not actually that bad in themselves compared to all the rest that goes along with them). Annoyingly, I have wide hips and carry more fat than muscle than if I were a man. I really hate those wide hips - I look in the mirror and see how much better I'd look if I had narrower hips (and I'm not saying that all women have wide hips, its just that women are statistically far, far more likely to have wider hips than men).
I had to watch my diet constantly even when in hard training because my damned hormones encourage me to lay down fat in the areas you don't want it to for running fast. Also annoyingly, I was 5 feet 2 inches tall, not a near 6 feet tall man. Even if I'd been an lower than average male height, for example, a nice 5 feet 7, I would have had longer legs and a longer stride length as a result. Most annoyingly of all, as I got older, I developed osteoarthritis, due to my female hip and pelvis angle causing my knees not to be as inline as a man's (an "acute Q angle", again far, far more common in women than in men). Plus, I always had to be careful where I was running, so that my emerging female body didn't get attacked by a potential rapist.
I know, taking everything into account, that if I had been born a man, I'd have been a lot faster runner. Its farcical to suggest that taking hormones for a few years can change height, bone structure and the benefits of spending a lifetime as a man and having the muscle memory of being faster and stronger. Suggesting that it levels the playing field is to me just another manifestation of male privilege and hatred against women, often by people who don't think sports should be separated on grounds of biological sex.
I love running so much, I'd change biological sex if I could. However, I know that taking testosterone for a year or two isn't going to change my bone structure, give me a narrower pelvis or make my legs suddenly 5 inches longer. I'm in awe of elite women athletes who run as fast as they do.