pitfalls of old age

I've been excited the last few months in that I've been able to increase the intensity level in aerobics class.  This past week I've had to drop down a notch or two (when I'm really tired my body doesn't always cooperate well) and have had the old pain in my feet and knees in the morning.  Disappointing, but I felt like I needed to keep pressing with exercise and weight loss in hopes of postponing Arthir the moocher.
So today, instead of using the exercise equipment upstairs (my usual Tues/Thurs routine), I went to a 20/20 class.  The description says by the time class is over you will have spent 20 minutes on each of the following: cardio, flexibility training, and weight bearing exercises. That sounds like something I need, so I head to the gym with a little fear, nervousness and excitement.  The first ten minutes in (while marching in place, the simplest of exercises) I felt a searing pain in my calf.  It's not uncommon for me to have short bouts of pain flare through my ankles and if I hold still for a few minutes, it goes away.  The pain eased up, but it never totally went away.  I have a nicely knotted muscle.
Given my past history with "athletic" injuries (I broke my foot in college tripping up 2 steps), I am more than a little humbled to admit that I strained a muscle while marching in place.  Why couldn't it have been a lunge, or a squat with weights or while using the blasted bouncy ball?  So since I'm prone to succumb to the "Fear of Man", I'll sheepishly admit how I aggravated the tension level in my muscle by saying "My body just has a mind of its own and it just wants to rest." :)