The word triathlon strikes fear into the hearts of us sedentary Londoners. Competitive sporting activity over 3 exhausting sports when the weather's limped from not very bright to downright miserable? You gotta be joking. And yet, Brockwell Park Lido's Aquathlon this weekend is almost beckoning seductively, teasing us with a 'number' of lengths followed by a 'gently undulating run' which finishes next to the cafe. It's like sport got inside a velvet glove and......