What I'm driving at is in general does a kind of blood that's really
good for enduro-sport also tend to have a somewhat higher risk of
clotting? And in particular if someone is living/racing at both high
and low altitudes (ranging from sea level to 11,000 feet several times
a year) is that doing anything to increase clot risk? Do hemo levels
relate to clotting?

Your pulmonary embolism (blood clot in the lung) after fracturing your
leg is an unfortunate but predictable complication of the surgery itself
(that's assuming you had surgery for it). All reasonably long surgery
under general anaesthesia carries a risk of DVT and PE in the
post-operative period (typically 10-20 days after the operation), but
lower limb orthopaedic surgery is _particularly_ high risk for clots. If
you didn't have surgery, simply the immobility of being in a cast and
not walking on it can be enough to predispose to a DVT +/- PE.

Living, working and training at high altitude tends to increase the
haemoglobin concentration, and hence oxygen carrying capacity, of your
blood. Your packed cell volume (PCV) or haematocrit, (which is just the
ratio of the volume of red cells to the total blood volume), will
increase, and hence your blood _will_ become more viscous.

Taken to extremes (erythropoietin overdose, rapid transfusion of packed
red cells, haematologic malignancies) the increase in viscosity can
indeed dramatically increase your risk of sludging off vessels,
especially the small ones. Simply living intermittently at the altitudes
you're talking about should not significantly increase your risk of a
venous thromboembolic event (VTE), though.

Taking half an aspirin a day if you're in your mid 40's and male is
probably good advice. It will _slightly_ lower your risk of VTE, but
dramatically lower your risk of a heart attack or stroke. There is
fairly good evidence, and some less so, of other likely health benefits
of the humble aspirin as well, if you're keen for some lengthy reading.

Kind regards,
Chris Cole