Hi KJ,
The verse says, in Heb 11:11 Through faith also Sara herself [u]received strength to conceive[/u] seed, and was delivered of a child when she was past age, because she judged him faithful who had promised.
Our actions, as the verse says, and which could, for instance, be applied to Noah building the ark just as easily, are a sign that we have believed what God has said to us.
As another poster has pointed out, Abraham had other children apart from Ishmael, by other women. He was not breaking any of God's laws through these sexual relations, and it is unlikely he ever stopped having them with Sara. I don't see what point you are making, actually, in mentioning Hagar, who did not replace Sara as Abram's wife, but was added to the number he already supported (with their children). Hagar, was special as a surrogate for Sara - Abram's ONLY wife.
I always find the biblical references to physiology utterly fascinating, as our generation seems to think it has the corner on such knowledge. Thus, the phrase 'received strength' is interesting, because of the differences between male and female reproductive physiology. Perhaps, like Hannah, Sara had a fickle appetite, or a hormonal insufficiency, which affected her fertility. Perhaps her great beauty made her rather aware of her physical image, and she resisted putting on an extra 15 pounds in weight, which may have made all the difference to her power to conceive. Also, it is possible for a woman to resist conception at a psychological level which affects her neurochemistry - in which case, 'received strength' could, possibly, mean [i]strength of mind[/i].
There is no doubt, also, that before God had spoken to him, both Abram and Sara were idol worshippers (Joshua 24:14), and infertility is a common problem for women turning to God from idolatry. It accounts for some of our modern day infertility as well.
Thus, it was necessary for Sara to believe God's word to her in her own heart. This is what God was after from her, and once she did, He honoured her with Isaac.
It is, also, extremely common when a woman has not conceived after many years of marriage, to assume that it will never happen, and to adopt a lifestyle, and a mindset, which are not conducive to conception. Who would ever know how many fertilised ovae had failed to settle in their wombs? In that we (modern science), still do not 'know' what sparks off a labour, it is fair to say that this is one miracle of creation, which God has kept firmly and completely, in His own counsel.
Remember Rachel - 'Give me children or I die'? This was a very similar scenario. Another very beautiful woman whose heart was not towards God. (Genesis 31:34, Genesis 18:15)