I wouldn't waste your zeals until you're 80+, tbh.
And I do think there are better uses for the zeals- like on strength/defence/attack/hitpoints. You get more experince for those.
If you really want prayer.. why don't you just kill the dragons for the bones and use them on a guilded altar? Or at least go gwd or something, make a lump of cash and spend it on the bones. It's a lot faster that way.