Mmm, there's a couple different ways I could go about making an analogy. I'll just ramble:
When I wrote it out, I was thinking like it was a chunk of rubber-material-- say you're using it all the time; it's flow-ey, easy to manipulate, use & expend, like water. If you don't, it sets, turns into more of a solid mass since it's all backed up and just sitting there. You have to learn to work it before you can use it. (for mana-draining, it would be like trying to draw gel through a straw.)
Alternative 1: Let's say it's kinda like blood-flow, or it 'heals' like a pierced-ear; if you're constantly piercing it, it'll be a lot easier to draw blood than if you have to start a fresh wound. (mana-draining is just creating the vacuum, not the wound, (which could open up magical research possibilities))
Alternative 2: Batteries. You've got a battery sitting there, if you're using it the contacts will be bright, shiney metal. If ya let it sit, the anodes get all corroded & junk. ('polishing' would take quite the effort, & require maintenance..)
I'm thinking of it more like a resource or material, not a muscle.
-On magic-users: They would be more vulnerable to mana-draining, but have that big 'don't go too far' mechanic of soul-breaking.
Meanwhile, unchecked, mana-draining/damage is unblockable & exploits a naturally lower bar- everyone and their mother would pick this, armies would be based around the concept & picking a pure-melee adventurer would be foolish.
Plus, mana-damage is usually an 'anti-mage' skill.