A quick look at the preceding verses lets us see some of the apostle Paul's heart. He states in these verses that he had learned to be content in whatever circumstances he found himself in. Although he gives very little detail, he says that whether he had plenty or had little it was enough. He also says that he was content whether his stomach was empty or full... and that these extremes were manageable because Christ in him gave him strength to face each situation.
The context of Phil. 4:13 is not just taking a dive off the positive side of things and trying to make every thing into a facade of ease. But rather it is saying that there's no room to stress; if things get tough it's His strength that will carry us through - and it will pass! And when things are going well - it's still His strength that will carry us through. And this too shall pass!
So no matter what we are facing today - both good and bad, He can effectively carry us through. Our struggle then is to learn how to rely on Him, not in the difficult times - that's when we call on Him most, but learning how to rely on Him in the good times as well. Rely on Him today - no matter what you are facing easy or difficult! Don't for get Him on the good days!!