Question about getting back into raiding

Hey folks

I stopped playing WoW mid-way through MoP and picked back up just last month (so about two years of not playing).

I'm on a pretty dead realm (it's crazy what can happen in two years) so I guess I'll have to look elsewhere - but I wasn't sure what I should be expecting when I come back to raiding.

For perspective, for the tiers prior to whatever tier ToT was in, I was usually downing bosses on heroic while content was still current for the patch. The best guild I was in was US #42 (Exordium, can't remember which realm at this point - Hellscream?) in T13.

I'm not using this as an advertising thread trying to find a guild, I'm specifically wondering, especially from guild masters, what I should be trying to get into to pick raiding back up.

Even though it's been two years, I've found that it was quite easy to pick back up on my rotation as a DPS warrior - my ilvl still sucks, but I'm trying to pick back up as best as I can.

I guess my main concern is that I'm still fuzzy on the exact differences between mythic and heroic, since I left right before mythic became a thing. I read up on it, but I'm not entirely sure how to separate what guilds at this point in the tier I should be looking at getting into.

Do you think I would be good trying to jump into a guild halfway through heroics or is that much too low or too high?

Thanks for any help.