BMI is Brain Dead, Go With Ideal Body Weight
The big impact of weight on running marathons? You'll add 2 seconds a mile or 1 minute a marathon for every pound over your ideal running weight.
By the way, BMI is not a good estimation of ideal running weight. BMI (Body Mass Index) says I'm overweight at 150 pounds despite looking svelte. Part of the problem is that BMI does not distinguish between men and women, body type (big or little frame), or athleticism (muscle mass weighs more than the fat it replaces).
Systems that measure ideal weight take all that into account. They might have you wrap your thumb and major fingers around your wrist to determine body type. You have a small frame if fingers overlap (as shown above), medium if they barely touch, and large if they don't reach.