You're very lucky to be alive. Get well.
One of the big reasons for clots even in more or less healthy people is not walking enough.
A lot of people nowadays drive rather than walk. Deep veins in human legs are an engineering wonder (wall structure, valves, everything), but even so, they are strained by the constant pressure of the blood being pumped so high up back to the heart. It's the price we pay for being bipedal (along with bad knees and lower back pain). Over the years the strain accumulates, especially if you don't move enough or have to stand for a long time. Even if you don't have any connective tissue problems and eat healthy, you can develop varicose veins. Varicose veins get disrupted blood flow, uneven surfaces and other issues that trigger clotting. Clots can and will travel up and clog vessels in the lungs (and other organs).
If a long, noodle-like clot fails to decide which lung it wants to mess up, it gets stuck where the lung arteries fork, and you just collapse and die instantly.
Add connective tissue or clotting issues many people have, for whatever reason, and bad veins in the legs become a ticking bomb.
Muscles in the shins do A LOT to support the veins and actually help pump the blood up as they move. Toned shins keep your deep veins healthy. All you need to do is just walk more.
It is also a very good idea to get your deep veins checked regularly, at least once every few years. It's an easy, cheap ultrasound check that can be life saving, but for some reason doctors rarely order such ultrasounds preemptively. You can fix the varicose veins in time and monitor any clots before they cause trouble.
sorry for the rant.