If its indoors on most corners you do NOT need to slow down or brake, the karts have some mean traction aslong as you have the muscle you can get em around most (tho maybe not all) corners fullspeed. Outdoor tracks is a bit different story.
Clipping the apex again is only really needed on outside track, on inside ones aslong as you can physically hold the line pretty much any line is so close it makes no difference.
After 50 or so laps you will be physical exhausted if its your first time.