I don't know about jogging if you're not used to that kind of exercise already. Maybe start a walking routine everyday for an hour or so (dinnertime is always helpful when you're fasting!). And situps ... hmmm, I did them while fasting, but then, I've been doing them for the past 2 years before fasting too. I can't see how doing situps would be harmful, though.
If you're low on energy, you can try Somer's cure: a tablespoon of blackstrap molasses for a pick-me-up. I drank peppermint tea with maple syrup in it and that usually got me going. Chamomile at night got me relaxed enough to sleep most nights. You should start feeling better as the days go on - I didn't really feel good until about day 12 or so my first fast (if I remember correctly - it was definitely past day 10). By day 21 I was actually enjoying not eating!
Listen to your body, rest when you need it, exercise when you can. Drink, drink, DRINK - especially WATER!!! Can't seem to stress that enough, I think. God bless and fast ON!