I wouldn't try using the heated water, because the fish have been somewhat slowly adapting to the cooler temp, and you would risk shocking them by rewarming too quickly.
When you get the heater, I would not increase the temp in the tank more than 2 degrees/day, so if it's still 68, start with the heater set at 70 and work up from there day by day unless they are showing stress before you start heating.
When you get the heater, I would not increase the temp in the tank more than 2 degrees/day, so if it's still 68, start with the heater set at 70 and work up from there day by day unless they are showing stress before you start heating.