Consistently Hitting a Wall ~8 Miles In

Hi there! I started running “seriously” (for me, AKA running consistently 4-5 times a week, usually ~20+ miles a week) back in like January. In my last two races, a 10 miler in April and half marathon yesterday, I will feel great until about 8 miles in, at which point I just hit a wall and don’t want to be running anymore and I start losing speed. Any ideas as to what’s causing this? I fuel early/often in races so I don’t think that’s a problem. TBH I haven’t really trained a goal pace for these races, I usually just tried to focus on getting “faster” without a specific number, do you think more mileage at pace would fix this?