I'd take the MiraLAX for as long as you need to. How to tell when that is, I don't know, as I'm still taking it daily myself I've tried taking it with and without food and it doesn't seem to make any difference for me. As Cheryl said be sure to drink a ton of water, though, because it needs water to soften stool and can also dehydrate you if your fluid intake isn't sufficient. MiraLAX might take a few days to take effect too.
I think a lot of folks here take Metamucil and/or Benefiber with MiraLAX so you should be okay there. I picked up some Benefiber but never used it, as I changed my diet to be super high in fiber (lots of veggie soup, some fresh fruit, fish/chicken instead of beef, granola bars and Wheat Thins Fiber Selects for snacks) and that combined with the MiraLAX was more than enough. I think there's a lot of variance here, some folks find that too much fiber and MiraLAX results in diarrhea, for others it's a necessity to get soft stools.
I tried sitz baths in the beginning with a $12 sitz bath outfit from the local drug store and it seemed okay for short term symptomatic relief in my case. It didn't seem to give any long term relief, though, so I personally stopped using it pretty early on.
Cheers and good luck, hopefully your fissure is just acute and doesn't end up going chronic.