75 y.o. Have clip-ons on my road bike and tandem - which has aero bars for stoker as well. I stretch, though it's not necessary for "the position." I mostly stretch to keep my knees out of trouble. I can always put two knuckles under my shoe soles. Too often when people say "core exercises" they mean like sit-ups or crunches, which don't do diddly-squat for cycling. Back exercises are good, but the best back exercise and the best stretch is to spend more time on your bike in position 2. Just do it. You're going to age, but you don't have to get old.

Besides stretching and riding, hiking and backpacking are really good for the lower back, as is taking long fast walks while rolling your hips, say 3-5 miles. My wife and I just got back from our annual 10-day backpack in the Cascades.