When I stepped on the scale in 2004 and saw that I needed to lose weight, I knew that I was not going to be able to do that simply by dieting. I did pick up the practice of portion control (I have been known to devour chicken wings at an alarming rate, which by the way is a bad idea the night before a wedding - especially if you're in the wedding party), which helped tremendously, but cutting out carbs or eliminating sweets just wasn't going to stick for me.
That's why I bought a bike (I had tried running in the past and without a specific goal, I gave it up quickly). It's so much easier to take thirty minutes to an hour out of your day to absolve the sins of snacking committed during the rest of the day than to strictly control your eating habits 24 hours a day.
As an added bonus, training for a marathon requires fuel. Before running the PittsburghMarathon last Spring, my appetite was voracious. I was training 40+ miles a week (tame by some standards) and I just couldn't seem to get enough calories in. I lost a total of about 10 pounds in the months leading up to the race, despite eating like the Tasmanian Devil. Being able to eat at Neato Burrito regularly makes for a great motivation to keep working out.
I'm not saying dieting doesn't have it's place, but when your focus is more on quality than quantity (because you're burning the calories) it makes it much easier to keep your weight down when you're putting in training time.