Are you taking measurements?
You are not exactly following the SCD "to the letter": you're only doing 3 meals a day (should be 4), and Tim especifically states that overexercising is one of the most common mistakes for starters. You might be loosing fat and gaining muscle weight.
Start measuring yourself for more reliable information! Also, if you can, get you body fat measured.
If you're already active and ate fairly clean before starting the SCD, you will not experience the huge losses seen here. Isn't that true with all diets though? If "Obese person A" led a sedentary lifestyle and ate 5000 calories a day and suddenly dropped to 3500 or even 2500 calories while adding a little bit of light exercise, wouldn't he/she see massive losses?
Without seeing exactly what you eat or your measurements, I'm gonna assume you're fairly fit. (training for half marathon? wow) Your body is already used to the amount of heavy activity and probably will see results slowly unless you're cutting a heavy amount of calories.
I used to be a gymrat, working out 5x a week and running almost as much as you did and never lost a single pound. When I cut back on my workouts and focused a bit more on my eating the weight came off. (a modest 10 pounds but I was pretty thin to begin with)
The first month or so is a fluke (mostly water weight from the upped protein intake. Measurements will accordingly shrink or expand with water loss so that's not a very accurate measure of fat loss either) so just wait it out and see.
I'm by no means a pro on this so here is my grain of salt.
- I think training for the marathon is really the factor causing weight to stay the same / gain. Especially since your measurements are stable.
- I read somewhere (From Tim) that for some reason, for the ladies, weeks 4-6 is when this diet causes a significant change. See if you can get through to there.
I've been on weight watchers with success before, so I know exactly where you're coming from. Good luck and try to stick to it another 3 weeks if you can.