Despite taking until she was 13 before, falteringly, completing her first unaided, self-taught strokes at the local leisure centre, the Dorset youngster finds herself, barely seven years later, on the threshold of her first major international appearance.
After a steady climb through the national age-group ranks, Karen, now 20, has been selected by England to contest three events in Malaysia - the 200m freestyle and, appropriately in view of her name, the 4x200m and 4x100m relays. In most sports, such elevation at so tender an age is regarded as downright precocious but in swimming, which has seen 15- year-olds win Olympic gold medals, it is almost a case of arrested development.
However, Karen, whose parents, older brother and boyfriend have no involvement in the sport apart from enthusiastic support, believes that this relatively late arrival gives her the chance of extending her career beyond what is normal at this level. "My long-term target is to compete at the Sydney Olympics in 2000 while the next big hurdle will be next year's European Championships in Scotland," she said. "Even though I've come to swimming quite late, I feel that I've got a lot of untapped potential, a lot of room for improvement. I'd like to carry on in the sport at least until my late twenties - the same age as Karen Pickering, or even older."
Beating Pickering, who is 26 and has been one of Britain's top swimmers for almost a decade, at the Games trials in Sheffield last month was perhaps the competitive highlight of Karen's career, especially as it secured her place in the team.
Under the watchful eye of the national coach Tony Watson at her club, Ferndown Otters, she has achieved a fastest time over 200m of 2min 2.14sec, three seconds shy of the British record and around five slower than the world best. Her daily training regime bears the traditional hallmarks of the dedicated swimmer's self-sacrifice - up at 5.30am for an hour and a half in the pool, an hour or so of weight-training at lunchtime, and a further hour and a half of swimming in the evenings. However, it's not all hard work - she allows herself four square meals a day plus regular visits to cinemas and discos.
And thanks to Lottery funding, which enables her to be a full-time swimmer, the main difference between this routine and the stoicism of the previous generation of amateur competitors like her idol, the former Olympic silver medallist Sharron Davies, is that she can go back to bed after her early- morning exertions for some extra kip. Her predecessors had to shrug off their exhaustion and haul themselves off to school, college or work.
Another recent improvement is the clampdown on doping in swimming. "Drugs do hurt a sport but it's good to know that people who take them are being caught," Legg said. "It gives the rest of us all the more reason to try our best."
There will be mild disappointment next month if she does not break the 2min 2sec barrier, even though she is well aware of the difficulties which the humidity will present. "It's going to be very hot so it will be important to keep drinking plenty. But knowing that I'm going to the Commonwealth Games is very exciting. I went to the World Championships in Australia earlier this year, but it was only as a reserve for the relay and I didn't get to swim. Travelling to swim is not a new experience for me, though, as I've also been to Canada, France and Italy with squads in the past."
Her next assignment after Malaysia is a spell of altitude training in a town called Flagstaff in Arizona. "Normally, we take the autumn off to recharge our batteries, but that's not been possible because of the Games. I've never done altitude training before so it will be interesting to see what effect it has on me," said Karen, who at 5ft 4in confesses to being "quite small for a freestyle swimmer".
The natural athleticism which saw her represent Dorset at cross-country running as a schoolgirl - before swimming was even a twinkle in her eye - has clearly helped her to overcome this mild disadvantage. That and enthusiasm, of course. "I might have started late, but I just love the thrill of the water."Reuse content