“that the perfect weight is the weight you are the day you P.B. in your event,” referring to the time you achieve your personal best — or fastest — finish.
-Tom Fleming (Slimmer Doesn’t Always Mean Fitter)
Good NY Times article about balancing training, performance, and weight. Lighter is better, but only to a point.
Fleming is a former 2:12 marathoner who won at New York twice and was a top ten finisher at Boston 6 times.