There's is something I disagree with here, the main thing.... You are walking/running... Implying that firebending is actually miraculously empowering a firebenders physical abilities. We know that their physical abilities can empower their firebending but not the other way around. If anything, bending exhausts them. Even in your example Azula wasn't touching the ground so all of her physical ability was used on bending rather than wasted on running. I disagree that a firebender would gain any physical buffs from fire or the sun. The sun empowers their bending, not their muscles.