Since the main thing is »lange Texte« I would recommend to put it first.
Lange Texte lese ich nicht gern.
Lange Texte lese ich [nur] ungern.
The use of »mögen« is also okay here (and my favourite), but you should omit the main verb in this case.
Lange Texte mag ich nicht.
In all three cases the meaning is the same and has no special connotation: I don't like reading long texts.