It's a great question. I start with one set of twelve different ab exercises. I do mentally count the eighty reps I do of each exercise and it's not to tough to count the eighty reps. When I finish my first set, then I do two more sets, counting each rep in each of the twelve exercises so I'm not counting to 3,000, but I'm just counting the eighty reps of each exercise. As I said, I toss in another 120 reps at the end of the three sets to even it off to 3,000.
The odd thing is that counting the 80 reps of each exercise creates a kind of internal cadence for me that lets me go into a zone that I find rhythmic and somehow comforting. In a strange way it's like being with an old friend.
With iron I do count my reps during my forty-five minute workout. Your way of going to exhaustion is an excellent choice, but it works best if you're trying to build muscle mass. At my age I've gained about all the muscle mass I want and I'm simply interested in flexibility and maintaining. Yes, I realize that if I don't periodically increase my weights that I'll begin to lose some muscle mass, so once in a while I'll add on more weights. But because my goal, at this point, isn't gaining muscle mass I just go to twelve reps of each intense exercise rather than to exhaustion. Heck, at sixty-three years old with a 12-year old, a 9-year old and a 3-year old that are my first children I'm pretty much at exhaustion all the time anyway.

Don't even get me started about having an Irish wife that's nineteen years younger than me. Well, actually it would be Kim who's at the state of exhaustion from being married to me...but that's another story, for another forum.
Btw, almonds are a real conundrum for me. I had a kidney stone eleven years ago and it was one of the worst experiences in my life. On the other hand a handful of almonds is one of the best things men can do to avoid prostate cancer. I long ago decided to take my shot and do the almonds.