Believers in Jesus are sometimes accused of 'blind faith'.  It is intended as an insult, meaning that 'you cannot see the problems, you cannot understand the issues, so you just believe without evidence ... your so-called-faith is therefore an excuse for not thinking'.  But that is not Biblical faith.  Real faith is realistic.  It fully sees the problems but refuses to allow the difficulties to overcome the confidence that God can do anything; and that He will always keep His promises.

Abraham was not blind to his aging body, or to Sarah's inability to conceive.  Neither was he imagining the impossible as a fantasy.  But everything changed when he heard God's specific promise.  The promise, combined with what he knew about God's character, was sufficient to strengthen his faith to the point at which he had no doubt that God would do it.  And so, God credited Abraham as being righteous, as a gracious gift.

You may say that Abraham was not strong in faith.  He was called from Ur to leave his family and travel to God’s destination (Genesis 12:1); but he went with his father and nephew Lot, and settled in Haran (Genesis 11:31).  When his father died, Abram (as he was called then) took Lot on God’s journey (Genesis 12:5).  The story goes on to be riddled with Abraham’s wrong moves - usually caused by him thinking that he knew the right thing to do when he did not.  But despite all that, he did trust God’s promises, and with every new step of faith he got back on track with God’s purposes (Hebrews 11:8-19).    

God has not changed.  He still has the power and looks for those who will give the glory to Him.  Of course, Abraham had a lifetime of making mistakes.  But as time went by, he practised believing what God said until he became fully confident in the Lord despite the impossibility.  It is the same for us.  God does not want us to be blind to the realities around us, because they are also a test our faith.  We must choose to place our hope in His unseen promises - rather than despairing because of the 'realities' of what we can see. It may be that you are facing that kind of test right now or have recently failed a test.  You can still get back on track with God’s purpose for your life if you will listen to Him, repent and obey.  The choice is either despair through what you see or hope through what you believe.  Make the choice and be fully persuaded that God can and will do whatever He has promised.