Someone asked me once if running was like riding a bicycle, in terms of "you never really forget how". My response was "Yes, if that means falling on your face and skinning your knee a few times until you get the hang of it again". While you obviously don't forget how to run, you simply can't jump back in where you left off. No matter how badly you want to. The mind may be willing, but the body just isn't able.
Of course, that doesn't mean most of us don't try. Many of you are probably guilty of jumping back in before you were ready. I know that I'm pretty much the poster child for such behavior :)
But there are some amongst us who are smarter. Patient. Wise.
They know not to beat themselves up for finding a once easy run, difficult. They know that before you can run long, you need to run strong. They know not to measure their success in mileage, but in effort.
And this applies to most all forms of fitness. You can't pick up the barbell where you left off. You can't expect to be as fast or as agile. You can only be present.
So don't beat yourself up. Own your experience and remember that hard work isn't measured merely by distance or pounds. It's measured in sweat and heart.