Honestly I wouldn't worry about it. I've tanked them maybe twice (and healed/DPSsed many many many other times), because I was nervous like you.
But in this case, just doing the dungeon beforehand makes the run smoother. I've helped tanks in Castrum who had no idea where to go or what to do; even when I had to tank it once I basically just winged it and copied what other people had done before.

The expectation is... in Castrum: huge pulls and have the cannons handle everything. Don't tank on the cannons or they'll break.
Praetorium: run past everything and only engage if you absolutely have to.

Just run them a few more times and you'll get better at it.