Another part of your long run training is to improve your body's ability to burn fat and to store glycogen. So by taking gels on your training runs your body will be burning the gels and not the fat, although you'll be using around 500 cals an hour running and only absorbing 240 so you'll have to burn fat eventually. I'm not exactly sure of the process of glycogen depletion but the more you use the body will adapt and store more. Again I'm not sure whether taking gels will hinder this at all.
I had heard the Amsterdam one was good and it would be an excuse to take the family away for a few days too (always works out better with the hubby if there are benefits for him too), will check that and Dublin out