Hi Lavster & welcome!
Apparently it's alright to exercise on a fast day (although running a marathon isn't recommended!) and various members here go running or workout on their fasts. It may be the sort of thing you'd prefer to build up to once your body has adjusted to fasting.
A lot of us have found it eaiser to save our cals for dinner on the fast day, thereby maximising our fasting time (around 24 hours if we go from dinner the night before) and having a nice big dinner with the full calorie allowance.
It's really up to you to do it whatever way works for you - if you find you're not hungry first thing then wait until lunch before eating. You may find you need to experiment a bit and see how it works best for you in terms of staving off hunger and feeling full enough and with enough energy to get you through whatever the day entails.