that is about right for runtime - because the mini lst has 2 motors it will drain the batteries quicker.
The best thing you can do is to get another battery pack to help keep the costs low for you. Over time, you can invest in a good charger to charge the batteries too instead of using the wall charger, that will give you a more accurate charge and give you a few more minutes of runtime.
You could invest in a brushless system, but that would be way too expensive ($200) an complex to get into right now.
So really your best bet would to get another pack of batteries, or even 2 more and down the road pick up a good charger like this one - charger
Oh, welcome to the forums, this is the right place!!!
On the racing note - most races for electric vehicles last for 5-6 minutes anyway.