Add a null to your comp, name it “control” and add a Slider Control to it.
For whatever property you’re animating, set a keyframe at time = 0 for the initial value, go forward one frame and set a keyframe for the final value. Then add this expression (don’t use the code from 2008–it got trashed when they updated the site)_:
slider = thisComp.layer("control").effect("Slider Control")("Slider");
sliderMin = 0;
sliderMax = 100;
myMin = valueAtTime(0);
myMax = valueAtTime(thisComp.frameDuration);
linear(slider,sliderMin,sliderMax,myMin,myMax)
The slider should then control the animation.