I wouldn’t eat if you’re full. You shouldn’t have any problems meeting your calories in 10hrs. A food-tracking app might help you get a visual on your intake. As for fluids, I’m a water and black coffee faster but people do different things depending on their goals. Find what works for you.
Don’t continue to eat if you’re already full, just eat if you feel like it. Drink Lots of water too around the clock, and no need to take electrolytes unless your fasting for more than 48 continuous hours
Don’t get too hung up on those multiday fasting things! You may get there, you may not… Just remember, it’s better to do six months of 14 to 18 hour fast as opposed to one month of killer multiday fasts… That just kind of break your spirit and make you give up! Go slow… You should be in this for the long run.