That could be explained if you start with an inefficient body. If you suddenly start running 5K races, you'll likely won't finish the first one, and it will hur

That could be explained if you start with an inefficient body. If you suddenly start running 5K races, you'll likely won't finish the first one, and it will hurt a lot the following days. As your body becomes used to the new activity, you'll start finishing your races and getting less drained afterwards. You're improving your fuel economy.