You do have to get use to swimming in a lake. It's different. Many people get freaked out the first few times. Maybe try to practice some more... just so you can get use to it.
Otherwise, stay to the outside, away from the bouys... and you won't get beat up as much.
Running... you only started 3 weeks ago?! Wow. Well, it takes time to get up to the speed and distance... so, if you need to walk in the tri... then you need to walk. That's OK. And, don't try to cram all the running into the next 2 weeks, you aren't going to make that much stride in doing that, and more than likely, you will wear yourself down. Just do what you can in the race.
No one is "over weight". Well... unfortunately... these races bring out the really fit people. For what it's worth... I'm 5'4"/125 lbs... and I feel self-conscious at races... I feel fat next to all those perfect bodies.
But, I also see other people who don't have perfect bodies racing... and a lot of the time they are kicking my butt. You can judge a triathlete by it's cover! Trust me on this one!
Can you register as an athena racer? If so... if there aren't any other ones... you will get a MEDAL for winning your age group! That will be cool! So, there is an upside to maybe not having a perfect body.
It sounds like you are going to do just fine. Just go out there, do your thing... and don't worry about anyone else!
Good luck!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"