I’ve gone into OP levels solo with 3 classes, plus 2 player with one of my daughters to OP2. Getting to 72 didn’t feel like a long process except for that very last stretch (70-72), where I caught myself watching the bars.
Honestly, a lot of what I did was focus on story quest progression through Tiny Tina’s and Torgue’s DLCs and the main game, farmed current Harold, Bee, and Lascaux every couple of levels, and set things up so that I’d be ready to farm at 72. For example, doing the ‘Cordially Invited’ string gives multiple turn ins for experience, gets you a good holdover weapon (Teapot), and sets up the big ugly varkid for farming so that you can go get a Baby Maker. Similarly, the Clan Wars quest has a lot of turn ins, a lot of killing, and sets up Slagga or Maggie farming for 72.
Mix in a little Legendary Loot Midget farming (for Bone of the Ancients) and Tubby runs in the Dust or Frostburn, and 72 winds up happening kind of on accident.