don't see anything wrong with adding mast....like everyone else said, gw will aid as well. my 2 cents: avoid tren....that shit killed my cardio/endurance on an epic level.
there's nothing wrong with adding mast but running a huge steroid cycle is not what your wanting from the endurance perspective... you also need to be at a body fat under 10% for masteron use and you need to run it much higher as well... 200 mg eod at the minimum... if you wanted endurance and an excellent all around stack for crossfit then test, eq, primo and sarms would have been the best option... here is the cycle you SHOULD be using...
1-16 test cyp 350 mg week
1-16 primo 600 mg week
1-16 eq 800 mg week