How does one get started?

I don't ride endurance, but I think the best way to start is to look for some endurance riders in your area. At least in Austria they are a very friendly bunch and don't mind helping novices. Maybe you could find an experienced rider to help you put up an exercise plan for you and your horse and coach you on the first few rides.
 
In GB the first thing to do is join the Endurance Horse and Pony Society or the BHS Endurance Society, They will give you a list of all rides and the qualifying rounds to do them. In GB you have to start with a 20 miler and work your way up to the 100 mile rides. Find out who in the US is the governing body of endurance.
 
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