You don't need to level as holy/disc to familiarize yourself with the healing spells you'll use at 60. Even shadow specced, you can easily heal instances up to say level 55-58 and you'll use the exact same spells.
In terms of levelling, both shadow and holy/disc rely on wand damage and Spirit Tap. Your rotation is almost the same except you swap Mind Flay for Holy Fire/Smite. The latter is just slower.
If you're truly set on going holy/disc, then get Wand Specialization and Spirit Tap, then work your way down the Holy tree until you've maxxed Spiritual Guidance, then spend the rest in Discipline.
Read my guide if you want more tips on levelling and healing: https://vanillapriest.wordpress.com
