Endurance athletes and increased risk of blood clots.....

From Canada
http://www.theglobeandmail.com/...19378004/?cmpid=rss1

Also
http://circ.ahajournals.org/...ent/128/25/e469.full

summary (of first link):
Dehydration and inflammation make your blood more prone to clotting;
Physical trauma like muscle strains can damage blood vessel walls;
Low blood pressure and a low resting heart rate allow blood to pool during
periods of immobility
And flow is further compromised when athletes take long flights to and from
competitions.

Prevention (from the second link)

Identify and manage any hereditary prothrombotic conditions through dialog with your healthcare
practitioner.

Be vigilant to thirst as an indicator of dehydration.

Avoid excessive consumption of caffeinated drinks and alcohol. Replenish both water and electrolytes during and after training.

Always plan and remember to take breaks to move/perform leg and upper body stretches during long
car/coach journeys.Wear compression stockings during long-distance air travel.

Avoid sitting in a cramped position and crossing legs at the knee and the ankle.Consider using contraceptives that do not contain estrogen (eg, the progestogen-only pill) with a family planning specialist.

Taking prophylactic aspirin may be a viable option, but always seek medical advice.

Most important, listen and respond to your body. Pain is not a barrier to overcome through greater endurance but an indicator of an underlying problem. If something does not feel right, stop training and seek medical advice.