I love training toward my triathlon goals, I am definetly a MOP with a goal not to finish to close to the bottom of the pack. I can say I'm very disciplined in my training and follow my plan very well. One of my primary goals with triathlons is to loose weight, then stay at a healthy weight. Triathlons are an awesome way to reach that goal.
The hardest thing for me to do is maintain a healthy diet. I doubt it applies to a lot of people here because we are all such finely tuned specimens :) , but sometimes for me passing up an extra plate, or a quick lunch trip out with co-workers to a not so healhty place seems as hard as running a 10k (or think whatever distance is hard for you). I know that your body will crave more when you work out, but its the choice of foods that is the battle for me.
I find it interesting how my will power can be so good with training, and so bad with food. I'm just throwing my thoughts out there.