Sam doesn't look over 40!
A check might be worthwhile Sam, and if all's fine then it's probably a case of some specific training. What you need to do will depend on the nature of the fatigue, but something as simple as doing some squats 3 mornings a week might help. As ever, start easy and build up the number progressively.
Another point we can forget is that exercise is usually on of several loads on us and if work, family or other areas become overwhelming then the fatigue does carry through into sport performance.
Also, I think there are some people whose muscle physiology is quite extreme and tends very strongly to either power or endurance with little crossover - I have a friend who is a phenomenal endurance runner, beating pretty well all then men in off road ultra races, but she is far from fast over 800 or even a mile.
Hope you're ok?
Mike