Here is what a doctor told me about ME/CFS:
Having chronic fatigue is like being in a yacht race without a keel surrounded by sand banks that you can't see. All the other boats are racing past to windward and yet you keep going sideways instead of forwards (that keel would've come in handy).
So you go further, pushing the boundries and then 'wham' suddenly you're stuck, crashed on a sandbank waiting and resting, hoping for the tide to change, so you can keep going, limping along without a keel.
The funny thing is you look like a normal boat, but you just can't perform like one.
I like this analogy. Has anyone else got a good one?