I have to agree with most of the posters here -- save your marks for level cap LIs.
It's a good idea to slot extra 3rd Age LIs as you level; this increases the total amount of IXP you earn as you kill mobs and turn in quests with IXP rewards, and it will help you get familiar with the legacies available to your class. (See lotro-wiki's LI page; the table at the bottom of the section shows how IXP rewards increase with the number of levelling LIs.)
It's also not a bad idea to go ahead and extract legacies where you can to put on the LIs you're using, but don't bother spending marks or shards for empowerment scrolls to get all legacies to max tier. This is for practice -- you'll want those resources at level cap to perfect your 2nd Age or 1st Age LIs, but working on a level 60 or 65 or 75 LI will help you know what you want when you do get to level cap.
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