Today I went out for a 20k run. I didn't get very far. After about 3k I was tired.
In fact so tired, it felt like I was in a constant uphill battle. Sadly it mostly a flat course.
I tried to take my mind off the sluggishness by wondering exactly how I'd describe how I felt. 'Running through custard' came to mind. The thought amused me for a while, about 2 minutes actually, then it was just hard again.
I managed to push through about 10k before falling back through my front door.
An old song popped into my head while I was out there, so I changed the lyrics a bit...
"Some days are diamonds, some days are like running through custard"