‘His running shoes were covered in mould’...what does this line from the passage signify?

Directions: Read the passage and answer the following question.

Nick, my partner, and Todd went way back. In their early 20s, they’d bonded over a mutual love of outdoor sport, riding mountain bikes, ripping into wild ocean swims, and pushing through exhausting triathlon courses, finishing drained but exuberant.
Now, a few years shy of 40, Nick was more accustomed to the comfort of a couch than a bike saddle. His running shoes were covered in mould. And his taste for competition had softened into appreciation for family life.
By contrast, Todd’s life had continued along the same athletic course. He took corporations all over the globe to trek mountains, ride across exotic countryside, and run marathons around the world.
During their reunion, Nick and Todd’s talks intensified and Nick’s eyes started flickering with ambition. On the way home he announced, “I’m going to run a marathon.” He had drawn key details from Todd about what to expect. A marathon pushes the human body to such extremes that the heart rate is elevated to maximum for up to five hours. The average runner finishes 2 cm shorter than they were when they began the race, as muscles deteriorate.
Even Olympians suffer. In order to complete a 42. 2 km course he needed “kilometres under his legs”. There is no shortcut to achieving this: he would have to train, train and train some more. Nick knew he was 10 kg (OK, maybe 15) above fighting weight. And because Todd had not sugar coated the reality – freely admitting to use of anti-inflammatories just to finish – he knew that a marathon hurt like hell.