NB: Please note that tango festivals are NOT Marathons!!! I encourage organisers not to mix up these names!
What is a tango marathon? As it sounds it is a long weekend of almost non-stop dancing.
What is the difference between a marathon and a festival? Marathons do not have live music, they do not have shows. They have a smaller number of people - usually balanced in gender, eg: 60 men, 60 women (or this is the idea anyway!)
Who is it for? Generally Tango Marathons are for advanced dancers who just wish to dance and are not interested in classes, shows and live music that you find at festivals. There is usually a limited number of people so there is more space to dance, and in general the dancers are from many countries and of a high level.