There is no greater form of avodah if you think about it - than giving up an expirience you desire, simply because it's the will of H-shem that you refrain from it. Even the most strenuous efforts to actively DO things, cannot compare to withholding ones-self from desire (especially one of this nature).
We see that notwithstanding all the avodos of the avos hakdoshim, and all the levels the reached, the yiddin still would not have lasted in mitzrayim, were it not for Yosef Hatzaddik, who laid the groundwork and made their survival possible, by standing up to the nesayon of: temptation in it's most extreme form vs H-shem's will. The Meam Loez bring down from the Yifeh Toar, that his agony was even more servere than that of Yitzchok Avinu at the Akeidah!!
If you think about it, it makes perfect sense. All nisyonos of both sur me'ra and asei tov, are just extensions of that first nesayon of the eitz ha'daas - which was a nesayon of temptation. And it is this nesayon which technically DEFINES a jew (the nation which has the role of rectifying the eitz ha'daas) as we see by Avraham, that H-shem singled him out from the rest of the world, to begin the history of the people who would carry out the name of H-shem through the generations, by giving him the mitzvah of the Bris Milah.