Truthfully , and I say this with no ill intent towards you whatsoever , I recommend two things ;
#1 , Get with that club Andy suggested to you ,
And ,
#2 , Forget all about any ideas you have of modifying anything at least till you've learned to fly it just as it comes out of the box !
The guys at Horizon who design/manufacture these things are pretty well educated on what it takes to make a good RC trainer airplane and right now in your case , in the Pilot/Plane link , the airplane in question isn't the weak part of the link , your lack of flying skills are . Again , this is NOT a dig at you , not a single person ever was born with the knowledge of how to fly an RC airplane , we all had to learn just as you will have to put the time in yourself to master the required skills , and doing so with an unmodified model plane that flies just as it should , should be your #1 priority .
PS , as to your thoughts of how to increase the flight times , I'll tell you this ; The only practical way to increase the flight time is to increase the battery's capacity , in other words a bigger battery . BUT ! , alas , a bigger battery that holds more of those precious electrons that keep your model in the air is also a HEAVIER battery , and adding any additional weight to a well designed RC trainer airplane will ruin it's docile flight characteristics . All other things being equal , making it heavier will require it to fly faster to stay in the air , and faster is the last thing a new pilot needs
