I bet most of us have known some whacko like this.
Oh my god I know EXACTLY what you mean. Years ago, I met a fellow, whom we'll call Kevin, in a 10K roadrace. Afterwards a friend and I got to chatting with him, and he told us about how he was training for the world champs in the marathon, how this was a tempo workout for him, etc, etc. He seemed nice enough and the prospect of having a elite runner to train with was enticing enough, so we all agree to meet again.
The first warning sign was when we all met for a training run a week later. It was a simple six miler, but Kevin had a really hard time keeping up. Like, we had to keep slowing down because we were afraid we'd drop him, and he'd get lost on the trails. At the time I didn't think much of it, because I have run with elites in the past who also ran slow in training runs, because they just tended to really emphasis the recovery aspect of those runs.
What was worse was what on first impression seemed to be the confidence of a talented runner revealed itself to be the delusions of a narcissist. The guy never could talk about anything other than himself. About all the great times he'd run (never in legit races, always in workouts) and how he was planning to get his Olympic trials qualifier "out of the way" at a local marathon which was notoriously slow and doubtful if it'd even count. He would talk about all the pro athletes he knew ("a close family friend" as he put it) and all the various trainers and nutritionists and chiropractors he had.
He claimed he was a professional runner and had major sponsorships. I came to learn he came from a wealthy family which basically subsidized this lifestyle so he never had to work. But he never made a dime from racing. Hell, he never beat me in a race, and I'm only a 34 minute 10K. His only sponsor (that I knew of) was Powerbar, and he just got free powerbars by claiming he was a coach.
He claimed amazing abilities, and then every time when he ran mediocre times, he had an excuse (ran too hard the first mile, took a wrong turn, got tripped).
Eventually his ego was his downfall as he started shooting his mouth off to other area sub-elites, about how he could beat them in a 5K. He was even challenged to a race with someone offering to pay his entry fee. He never accepted. He alienated everyone, and wound up training with a group of hobby joggers who were naive enough to believe his stories.
The last I heard of him was when, after a local race, he took to a message board claiming how he was set to win the race in "mid 15s" until he was tripped (again). This was too much for me. I was in that race, and saw him get tripped, and based on my pace at the time (it was a 5K 10K dual, I in the latter, he in the former) he was racing 17:30 pace at best. He woudl've had to do low 4:00s the next two miles to run the time he claimed he was going to. I said as much in the forum, and soon after he blocked me on FB and I never heard of him again.
I think he finally quit running, but boy there was a while where he was the stuff of infamy in the running community. Everyone seemed to have heard of him and he was the source of a lot of jokes and sad asides (even now, years later, he'll still come up on conversations - "Remember Kevin? What ever happened to that guy?"). I'd never before (or since) seen someone so full of delusions of grandeur, so convinced of their own greatness, and so wholly unable to back up the boasts. It was really, really sad.