I'm soon to be over 30 and I weight train every weekday. Funny thing is it's only something I've started in the past 2 years. I'm really glad I did though. I feel a lot better in general and have more energy.
Looks like pipe B is your drain/vent. You can put your drain there. Just make sure there is a trap to prevent sewer gas from coming up through there.
Pipe A is your toilet drain.
I'm still pretty bummed everytime I see this thread...
Maybe you could rinse the rock and sand and reuse it? Let it cycle for a few months and jsut slowly add to it???
Heck I'll even send some frags your way.
If you use an adjustable union in your riser pipe you won't have to make adjustments. You can just slide the durso up and down as needed to control water level. Here's a picture of mine.