When you are starting to
use muscles that you haven't
used before, the fat
oxidative capabilities
of those muscles (
using oxygen to burn fats) is quite lower than the surrounding muscles, so your body has to rely more on (1)
sugar, and (2) particularly anaerobic
use of that
sugar: in order to
use muscles for endurance, your body has to create a lot
of infrastructure, such as capillary networks, in order to get fat and oxygen to those muscles.