I'm no expert, but I "came back" myself (to trombone, that is) 15 or so years ago. I had the good fortune back in college to have studied with the bass trombonist of the Dallas Symphony Orchestra at the time. He had just made the comeback after being completely off of the trombone for a year for very major oral surgery. He told me what he had done: starting on F in the staff, and staying pretty close to that range at first, he played long tones, holding his embouchure even when he'd stop to breath (breathing through his nose). Bit by bit, he'd add range, but always held the embouchure for each note while he breathed. I took the same track when I started back up after 8 years away, and found that it took me about 3 weeks to be good and comfortable from F below the staff to F above it. Progress went more slowly after that (to be honest, I was able to really play some stuff with that range, so I quit using that technique and started playing tunes

), but within 3 months or so I was secure* up to the fourth ledger line Bb.
I wouldn't suggest laying off for a couple of days even. Just stop for a while (an hour, maybe?) when you get tired. Take it like you would any other physical training. Start simple, and get the technique right. Then push yourself (not your mouthpiece

), but back off when you begin to feel pain. You'll be back to your old form before you know it!
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Joe Baker, who also suggests lots of MP buzzing between practice sessions -- focusing on proper embouchure form.
*secure -- by which I mean I can consistently pick it "out of the air" by feel.