I'm planning to use it as mulch or compost it.

Won't that affect the chemistry of your compost bin? How would you know after that point if you need more browns or greens?

Path material.
But I'm a composter so I would treat this as a green due to 5 & mix it with all the other stuff when I build my hot batches. The 5 will be 1 by the time the bin is finished. The unfinished pine needles would be excellent mulch.
Have fun & experiment with this 5-1-1 mix.

Spread it on your lawn, spread it on your garden, wherever you need some additional organic matter. Or cover it, to keep it dry, and save it for next year.

Did you add dolomite lime to your 511 mix, some swear by mixing a batch and letting it set for a few weeks so the lime could work bringing it to the proper pH. I just thought i would remind you in case you forgot.

As far as reusing it, if it still structurally sound, you could use it again(care of salt accumulation, probably best to flush it thoroughly with water before re using. After it losing structure, you can use it is mulch or in the compost. If you dont have huge amounts of 511 mix, it isn't going to do to much. If you are that concerned, you can add grass clippings, or other nitrogen to combat the high carbon 511 mix. But also note, if u are throwing it in your compost, it most likely already lost its structure(decomposing) and all the ferlizer you ran through it, it probably would be a decent ratio to compost.

It's not that I forgot, its that I read somewhere that you should not prepare a soil mix (like the 5-1-1) until it is ready for immediate use.

But what you're saying does make sense (to me at least). The pH doesn't just get adjusted instantaneously right?