Posted: 10/06/2014 at 17:22
I try and hit 'race weight' for important races but it's not a weight I can sustain long term and be healthy and train well.
I do this too, maybe 2-3 times a year I'll bother to drop the extra ~2KG above racing weight, that I tend to carry when training normally and treating myself to some red wine and chocolate when I feel like it.
This seemed to work for my marathon and HM PBs. However my fastest of four 10K races so far this year was actually at my heaviest - mainly because it was the best weather of the lot. ie once fairly lean and fit, all the other variables on race day are much more likely to make a noticeable difference than 1-2KG either side, so whilst it might help you'll never know for sure.
But Tom you're talking about losing 16lbs in 3 weeks, which is equivalent to 56000 calories in 21 days, so a calorie deficit of around 2600 calories per day. That's not possible to do healthily, especially for someone already at a healthy weight. The 5 pounds lost already is probably mostly your dwindling glycogen reserves being used up and the associated water weight that will have been shed with it, it's extremely unlikely that you would have lost 5 pounds of fat within a week unless either by starving yourself or by eating normally and running a 20miler every day!