restinglawyer I can't really help you there, because I didn't take up serious exercise until after I started low carbing! But I can tell you that, aged 54, I started running (started with C25K) and did manage to run 5K in under 30 minutes.
Now I train with a trainer twice a week, and I always train in a fasted state (unless I see him in the afternoon), and never have any problems. And let me tell you, he makes me work very hard!
Eva you can have sugar-free jelly, but go really carefully with that stuff. Firstly because you don't want to suddenly get lots of sweet cravings again, and also because there is a school of thought that your body responds to artificial sweeteners as if it's sugar.
Reggae - take some paracetamol - don't suffer! You need lots of water, but also make sure you're getting your electrolytes, in particular sodium - try a hot drink made from Oxo/Bovril/Marmite, and make sure you're cooking with salt/adding it to your food.
Mothership try shifting most of your water consumption to earlier in the day. But you will get used to it, I promise! And stay away from the cake ...
... and trixy - you and Mothership need to do some serious reading about low carbing if you're doubting it! Please, please, please go and have a look at the spreadsheet and at the links there. There is no link between eating saturated fat and coronary heart disease. And eating fat will not make you fat.