With the help of repetitive training, we are always, if unwittingly, working on our Speed.
If an opponent is allowed to get within striking range, the defender loses the advantage of controlling the distance and must rely more on speed and reaction time. To some degree, strength and endurance training will help to increase speed, but it should not be the ‘go to’ for technique since, from a self-defence perspective, speed and power naturally diminish with age. While training hard and sweating a lot can be good, it needs to be closely monitored and evaluated so that poor timing and technique are not ingrained alongside speed and strength.
That being said, it probably makes more sense to train in something that one can get better at, even with increasing age.
We train to improve the ability to control distance and timing, to aim always to experience those sweet moments when little effort is used to astounding effect. As well as being part of the joy of learning martial arts, it can often seem like magic to the observer, as if the movement was too fast for the eye to see.