Gravel is harder than asphalt. Find a way to extend your commute home (when you have more time) to maybe 10-15 miles. That'll get you additional training miles.
General rule of thumb, on a not too hot or humid a day, is 20 oz. water bottle per hour, YMMV. Add the 3rd bottle cage, maybe find some of the big Zefel 32oz. Water bottles, or at least the CamelBack 24 oz. insulated bottles. I personally would be wearing my 100oz CamelBack pack. Plus 2 water bottles