Are you having it done at the GP or hospital?
I had one done aged 25 that was an absolute doddle - as in on the table, frog legs, bit of pulling, done and off to do the shopping and pick the kids up from school - but I had another specifically for adenomyosis a week ago.
The second was a world away from the simple GP visit (local anaesthesia is not a walk in the park, but they said it was necessary due to the adeno/it would have been far harder without it) and all I wanted to do afterwards was crawl into the corner of the sofa and feel sorry for myself for 24 hours.
I think it's definitely having a positive effect now after a week as I feel less swollen, but they also told me to keep taking oral progesterone as well for a month to minimise the chance of bleeding. You should also be told no tampons (or anything else) up there for a week afterwards. Also - take painkillers before, make sure you're well hydrated/have a sugary drink with you and bring an old fashioned, thick, supersoft pad to wear with period pants, whether they're fabric or a disposable pair; there may be some bleeding, especially with everything being so inflamed in adeno and you may feel sore.
I'd also say if possible, get a lift home - public transport was not fun and I can't imagine driving immediately afterwards is a good idea if you have the cramping and tenderness I had.
The best thing about it was the sheer fluke in timing the appointment that it wasn't right at the point of another heavy bleed - and of course, if it does the job it's intended to do.
Basically, a Mirena insertion when you have a healthy uterus, cervix and lining is a lot different to when you've got adenomyosis. That's not a reason to avoid it, but it's just not as simple.