Often times a big space between nodes means that your plant is stretching for more light. What kind of lighting are you using and how far away from the plant is it? Although WW is a hybrid that I believe has about 60% sativa in it, they tend to be pretty bushy plants.
The cause of the flimsy stems may also be a lack of a breeze in your grow room. Do you have any moving air on your plants? Putting a fan in the grow room causes the plants to put more energy into a stronger stem. This is a good thing, at least up to a point. Too much wind will cause too much energy to be put into strengthening the stem and not enough into the rest of the plant.
Was you plant/soil dry when you tried to tie it down? I could see this being a possible cause as to why it snapped as well.
side note: A lot of people that grow WW, myself included, burn their plants the first time around by using a nutrient solution that is too strong. It is a very nutrient sensitive strain. Many people suggest that, in general, you should start with a nute concentration of 1/2 or 1/4 what the package says. I would take that one step further and start with 1/6 the recommended concentration for the first feeding and work my way up from there. My plants did very well with very little feeding throughout the entire grow.