De or het marathontraining?

The correct Dutch article is:

de marathontraining

Rule Words ending in -ing: almost always de. See the rule

How to use marathontraining

the marathontrainingde marathontraining
this marathontrainingdeze marathontraining
that marathontrainingdie marathontraining
our marathontrainingonze marathontraining
your marathontrainingjouw marathontraining
every marathontrainingelke marathontraining
which marathontraining?welke marathontraining?
a big marathontraining / the big marathontrainingeen grote marathontraining, de grote marathontraining