This is what the wiki Winter Grepolympia says about training troops:
Training
For each discipline, you have an athlete that you can train. In order to train him, you need units (land or mythical units) to assist. You can rely on any unit from any city to help with the training, but be warned -
Units used for training will be permanently taken from the army.
The Training Grounds offers 2 free training slots, and another 2 can be purchased for 100 and 200 gold respectively. These slots remain active throughout all disciplines. The number of units that can be trained in each slot is limited, and this results in the training time being 1.5 hours per slot.
Once you begin training your athlete, you cannot cancel it and you cannot get the units back. Think carefully whether you wish to use that unit. Each unit used will provide training points for your athlete, and each unit provides a different amount.
https://wiki.en.grepolis.com/wiki/Winter_Grepolympia#Training