The most useful analogy for me is that it is similar to lifting weights. You don't go to the gym and just hold a weight in tension for 60 minutes. You lift for a moment and relax for a moment, over and over and over. In meditation you're not (often) purposely "letting go" the way you do while lifting, but your brain will do that anyway. That's fine. The important thing is you engage again. Each time you detect your attention drifting and you bring it back to your practice is another "rep" of the workout. Hold it for as long as you can, and when you detect it's drifted again (as it will), you've already started another rep!