Hi Krys, welcome to the wonderful world of TE.
I would second everything that roadie gal said.
Also, rest assured in your swim, just relax and have fun in the water. This is where most new triathletes have the most trouble so you've already gotten that beat.
Biking legs come around faster than you think. Especially if you already have some endurance training like running. I did my first tri on some regular mtb training of 1 to 2 hours at the most and one ride on my new bike at the full 40 km distance (my first tri was an OLY). It was fine. Just do what RG said and keep your effort level where you are working but not killing yourself. You will be able to finish.
Running off the bike is hard. So practice the transitions. Give yourself permission to walk during the race. Everyone walks during their first tri. Enjoy the atmosphere and then if you can, push hard for the last mile home.
You'll do fine. Training all the disciplines on different days will get you across the finish line. Don't worry about if you can do it all in one day. I know it seems impossible, but really you can.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS