"Practice is not fun."
I'm sure the usage of omoishiroi over tanoshii is due to the context. No one, even if they find it enjoyable, is having the sort of fun studying that they have at a party or playing a game. Tanoshii is for activities that are more joyous in an emotional sense and not an intellectual one.
楽しい is more of a joyous feeling, feeling happy, fulfilled, entertained, most commonly used to described an activity, such as "that was fun", "that was enjoyable".
面白い is more of describing the nature of an activity as interesting, fun, that it attracts your attention, and while used in its negative form, 面白くない, means boring, uninteresting.
Another similar word that gets many Japanese learners confused is 嬉しい vs 楽しい. But this deserves another thorough discussion elsewhere.
That's what I said and I got it right. In other threads people are saying that "tanoshi" translates more to the fun you have at parties, so in this case: practicing is definitely not as fun as the fun you feel at parties. Tl;dr: yes I think so, but it affects the tone of the sentence.