This is cool. I love the idea of gym iron in Magic as equipment with an exert and +1/+1 counters theme.
I personally think the "next time the creature untaps" clause could just be be changed to "activate this ability only as a sorcery" instead as it cleans up some memory issues and brings the power level to a more acceptable level considering the mana costs. I also think it makes more thematic sense - it gradually pumps up the creature instead of serving as a combat trick (plus squatting 500 pounds at instant speed seems odd).
Edited the card in the OP to fix problems with memory. But the addition of +1/+1 counters really should be delayed, since one actually gets weaker during exercise, and doesn't emerge stronger until recovery occurs over the next day or two.
As written, the card allows multpile activations, as you can tap and exert tapped and exerted creatures. It should be:
Equipped creature has " T, Exert this creature: ..."
I personally think the "next time the creature untaps" clause could just be be changed to "activate this ability only as a sorcery" instead as it cleans up some memory issues and brings the power level to a more acceptable level considering the mana costs. I also think it makes more thematic sense - it gradually pumps up the creature instead of serving as a combat trick (plus squatting 500 pounds at instant speed seems odd).
Equipped creature has " T, Exert this creature: ..."