Various achronim discuss the kasha, why did Avraham not give himself a bris mila before Hashem commanded him to. If Avraham kept the whole torah, he should have given himself a bris right away.
There are a number of interesting answers to this kasha.
1) Brisker Rav: The classic Brisker answer is that Avraham didn't have a "shem oreil" until the command. If he didn't have a "shem oreil" then there would be no reason to give himself a bris
2) The Hafla'ah in Panim Yafos writes that there is an issur chavalah on a ben Noach. As long as there was no command to give himself a bris, it would have been chavalah to do so. Therefore, Avraham had to wait for the command.
One could ask that Avraham could have done the mitzvah as an aino mitzuva v'oseh and that should be enough to remove the issur chavala. L'chorah you have to be say that Avraham had dinim of a ben Noach l'chumrah and he had to be machmir and not be oveir the issur chavala even though he could have done the mitzvah as an aino mitzuva v'oseh.
3) The Sefas Emes writes that the sod of mila was so great that Avraham didn't understand it until Hahsem explained it to him. Using this S'fas Emes perhaps we can say that the reason Avraham didn't do mila is because he wasn't even aware teh mitzvah existed. Every other mitzvah Avrham figured out on his own. When it came to mila, Avraham couldn't figure it out until Hashem told him about it.
4) A saw another chassidishe answer similar to what I wrote in #3. Each mitzva is k'negged an eiver in aperson's body. However, when it came to mila, the orlah blocked the kedusha of that eiver and prevented Avraham from realizing there was a mitzva of mila.
5) Mila is a k'risus bris. A bris needs two parties. If Avraham did the mila on his own, there wouldn't have been a k'risus bris