Even if you're the strongest and most determined person in the world, time will come and your body will feel weak. Maybe it's because of overworking or aging a little bit. It is normal, don't worry and don't feel threatened.
Your strength will only be there for so long, it will not stay forever. So you have to work hard while you are still young. You have to make the best out of our free time.
If you are experiencing weakness, all you have to do is keep moving. Even if you are slow, just keep moving, you will still finish even if you are not fast anymore. You can still reach the finish line, you can still reach the top of the mountain.
You may weaken a little bit but your strength will never ran out. You can still find that energy that doesn't exist. There is no way you can't move anymore.
Never let your momentum die because once you stop... you will never be able to make a comeback anymore. You will feel good about resting, you will be satisfied of the comfort that you are experiencing.