I think the key here is making sure your electrolyte balance is good. Especially salt. People tend to supplement salt and over supplement everything else IMO. If you're very active I don't think 2 to 3 teaspoons a day of salt is out of the question.
Physically, I'd say I'm at about 90% most of the time. Tend to hit a lull sometime between 1100 and 1300 when the morning jolt of my body kicking me out of sleep is wearing off. Then I usually hit a second wind sometime after that and I'm good until 2200-ish. Also, it should be noted I use no coffee or drugs/supplements with wakefulness.
Mentally the energy levels are literally off the charts. Focus is especially improved.
I have been exercising. It varies. Some days are really intense, others very light. The last 3 weeks or so I've taken to doing bodyweight exercises every day. Push-ups, Sit-ups, and Squats. Up to around 50 of each daily now. Started running on Sunday to mixed results. It's much much harder than I thought it would be.
The only thing I would worry about is if you're not already fully fat adapted, and I mean FULLY. You're gonna have a real rough first week to week and a half if you decide to fast that long. Even with that, I say go for it. It's rough, but it's worth the experience.