Of course it is a concern coming dead last, to the point where a note was made of it. If Spencer's tank is an issue, which is something we've suspected all along, then he'll struggle to play games in this day and age, with the limited rotations.
That said, the 2km run probably isn't one he needs to excel at. He'll be looking to do well with repeat sprint drills or something along those lines.
It is a concern if we're questioning the tank of both White and Lee. We can't carry both.
A "tank" isn't a physical attribute though. Its fitness training. Pure and simple hard work. Some kids struggle to endure the training needed to improve though. But just playing footy at a high level year after year should give him some level of improvement.