Jim Ryun was an exceptionally gifted mile/1500 meter runner. He is arguably the most talented high school miler of all time. But that was in the 1960s and the sport has evolved over the past 60 years. Predictions of what he could run today are subjective. He would be one of the best in any era. Yes, he would be running in the 3:40s today.
He was the first high school runner to break 4 minutes in the mile as a junior in 1964. He made the Olympic team that same year in the 1500 going to Tokyo. The following year 1965 he won the national title in the mile running 3:55.3 and outkicking Olympic champion Peter Snell.
As a freshman at Kansas in 1966 he broke the world record in the mile running 3:51.3. This was two and half seconds faster than the existing record held by Jazy of France. Ryun was well over six feet tall with long strides, but his turnover was exceptional for his size. He would anchor the Kansas 4 x 440 relay and had a split of 46.9 at the Kansas Relays. He consistently ran 47 second 440 relay legs. He broke the world record in the 880 in 1966 running 1:44.9. His finishing speed was unmatched at the time.
In 1967 Ryun broke the world record in the mile running 3:51.1 and crushed Herb Elliot's 1500 meter world record running 3:33.1. But he was starting to wear down. He ran heavy rounds of interval training in high school and college, but the science of recovery was not well understood. In addition, he had known nothing but success and put a great deal of pressure on himself. I don't believe he was properly prepared for handling failure and disappointment by the coaching philosophy at the time. I had a Division I college distance coach who's hero was Vince Lombardi, the hall of fame professional football coach. Rest and recovery was for weaklings.
In 1968 he won a silver medal in the Olympic 1500 at Mexico City running 3:37.8. He lost to altitude-born Kip Keino who ran the race of his life. This performance is underrated considering Ryun was born in Kansas and this race was run at over 7000 feet. By his senior year Ryun was burnt out and it showed. He raced, but was losing to up and comers like Marty Liquori. He made the Olympic team in the 1500 in 1972, but had a tragic fall that knocked him out of the final. He probably would have outkicked Pekka Vasala of Finland in that Munich final and won the gold medal. Afterward, he ended up turning professional and ran on the "pro" indoor circuit in the 1970s (Google that history). There was no money in the sport at that time for "amateurs".
What made Ryun special was his natural fast-twitch speed that allowed him to accelerate and finish just as fast as today's best. Better training both physically and mentally and the financial sponsorship to allow more patience would make him special today.