If you sit for many hours, like on a cross country or cross ocean flight, or because you are a dev or gamer with tremendous concentration and focus, the blood quite literally just sits in your legs and eventually starts to coagulate. It doesn't matter how much exercise you get the rest of your day, those 6 or 8 hours of sitting can quite literally kill you through the creation of a blood clot that then flows through your system and blocks blood flow to the heart or brain (all learned personally when I developed such a clot from routinely sitting long hours in a coffee shop with my laptop and 100% focus).
If you're like me and can focus easily for many hours straight, get a Fitbit, turn on activity tracking, and when it tells you you haven't walked around recently get up and walk around. It's much less impactful to your concentration than an emergency ER visit followed by a lifetime of prescription blood thinners to reduce the now much higher risk of a second clot.