I think the big issue would be the replacement steroids. You need to replace and having had Cushing's I can tell you from experience that large doses of steroids (the cortisol type) tends to destroy muscle and now that my adrenals are out and I have AI, I can tell from experience that replacements are not perfect and can't replace like a normal body.
That being said, you are going to have to work within the parameters of your body as it should not be impossible but it will take knowing your body and having someone with you that can also recognize the signs of crisis and get you the fluids, salt or meds as you need them.
You should experiment with your doctor's guidance on how to stress dose, what salt/fluid intake you need (no water to wash salt away, only salted water, for instance may work better for you) and find a regime that allows you to not go into crisis both during the event and after. I mention after as many of us notice we get through things only to crash bad after.
Are you on a short acting replacement aka hydrocortisone or long acting like prednisone?
I am on 15mg of Prednisone a day with the fludrocortisone of .01 mg. Basically I was really big into weight lifting before I had addisons and I see a difference. Before I would be able to push out a few more reps before being done and I would know I am almost done cant do anymore. Now its like I either can rep it or I cant, there is no feeling of im about to be done. I understand that I need meds, salt, etc but it seems they are holding me back physically.
15mg pred is 3x the normal replacement. Any reason why the doc has you on such a high dose? Do you have other disorders and need an anti-inflammatory?
Being in such a high replacement is a double edged sword as I mentioned before... It can jazz you on cortisol but it can also literally eat bone and muscle (and eye tissue and more, cortisol just is a destructive puppy of a med)... So if you are trying to build muscle and are on a high dose... You won't be able to, in fact, exercise will or can be be rather destructive if your cortisol is in excess.
So, you need enough cortisol for the stress of exercise, but heck, you are taking a lot (from a patient point of view) and long term, it adds up to damage. I take an even longer acting steroid and it is awful.
Why 15 and how do you take it?
My mistake I take 7.5mg of pred, 5mg in the morning 2.5mg afternoon. My ACTH levels have been really high so thats why my doctor has me on what is the equivalent of 30mg hydro. I do better with the pred than I did hydro.
Everyone "does better" on high dose steroids.. it is easier, you have "energy" but you will pay a price and you as a body builder will pay a bigger one as it will be harder to build muscle.
You may want to experiment with your doc's permission how to mix high and low dose so you don't destroy muscle and bone.