Aspected Benefic should be used primarily for the HoT (or Shield if in Noct sect later on) but you have to remember it has a frontloaded heal on it and is proportionally more expensive than Regen on a WHM.

When I heal as AST I tend to go Diurnal but I don't keep the HoT ticking like I do on WHM.

Remember you do have some options as you level that can help alleviate MP issues as well. In dungeon content specifically you've got the Bole card you can throw on the tank to mitigate some incoming damage, you can also use Ewer on yourself if you get low on MP. Obviously these aren't optimal uses of your cards in a perfect world but sometimes in pugs it's safer to ensure longevity than huck Balance at a dps and hope they can burn things down before you've exhausted your resources.

Later on you'll get a move called Celestial Opposition that extends any buffs you've cast in an area around you. You can use this to extend cards but also regens you've placed (you have 3 by 70) and even the Refresh effect of Lucid Dreaming.

It's important to remember when going from WHM to AST that AST just doesn't gave the seemingly limitless MP of WHM. You can and will go oom even at 70 if you don't use your tools well.

I'd also toss in some advice not directly related to your mp management but something that can help it - understanding your party. AST benefits the most from knowing what the other people in your group should be doing so that when you can buff someone you're getting the most bang for your buck. There is no point throwing a Balance on a person who is single targetting in an AoE situation, for example. Whether you come by this knowledge through just always running a parser (so you can buff the highest dps) or the more roundabout method of estimation from watching your group for key moves or indicators of performance (RDM hardcasting 5s spells, physical dps at full TP on an AoE pull) you'll see your cards go that much further if you place them on stronger dps. Things dying faster means less healing for you to do.

This is my favorite AST guide. I hope it helps you as you learn the class! Good luck!

http://dtguilds.enjin.com/davidsastguide