A few years ago when I was in the middle of marathon training I weighed in at 137-39 and I'm 5'11". When I wasn't in the middle of hard training, I would typically weight around 142. I found that once my weight went under 140 I would frequently get injured, which seemed counterintuitive at the time (less weight means less pounding).
Leading up to my last marathon, I was running more miles than ever, but kept my weight around the 142 range. Previously it was always my goal to get as light as possible, but this time I consumed more calories in an attempt to keep my weight stable and hopefully stay healthy...I ran a 5 minute PR.
I think each person's ideal weight is different, it was something I learned by trial and error...but the mid to low 140s seems to be the sweet spot for me.