Nothing like seat time for track fitness. I've been semi fit in the past but still fatigue badly at the track due to lengthy spells between days. I'm sure Rob won't mind me saying that someone like himself with generous proportions can still be super fast on track but might struggle doing say a beep test.
Of course combining the two, plenty of track time and and a healthy life style of fitness and diet....win/win.
I struggle with all the above.