From the article:

Muscle tissue can remodel itself, which is one reason why exercise becomes easier when we do it regularly, Lessard says. Over time, aerobic exercise such as running or swimming can alter muscle fibers to become more efficient at using oxygen during exercise. “We also grow new blood vessels to allow more oxygen to be delivered to the muscle, which helps to increase our aerobic fitness levels,” she says.

The scientists propose that high levels of blood sugar may prevent muscle remodeling in part by modifying the “extracellular matrix” proteins in the space between the muscle cells, where blood vessels are formed.

Adapting to aerobic exercise as though it were strength training:

The scientists found that these JNK pathway signals were getting crossed in the hyperglycemic mice, by activating pathways associated with strength training, even though the mice were performing aerobic exercise. “As a result, the muscles of hyperglycemic animals have bigger fibers and fewer blood vessels, which is more typical of strength training, rather than aerobic training,” Lessard says.