MiraLax doesn't work quickly; it can take as long as three days. Despite its name, at the recommended daily dose (1 capful dissolved in fluid), it's NOT a laxative; it's a powerful stool softener that can take days to work, but once you continue to take it daily, it continues to ensure that you have soft stools (Colace is scarcely more than a placebo when you compare it to MiraLax.)
That said, I'd never recommend that someone take any new med they hadn't tried before if they knew they had an important meeting the next day.
Polyethylene glycol (PEG), the active (and only) ingredient in MiraLax, is also the active (and only) ingredient in the bowel-prep GoLytely. The only difference between them is the absolute dose of PEG. GoLytely contains many, many times more PEG as well as fluid, and as such has a completely different action: it causes explosive diarrhea shortly after you ingest a significant quantity of it. That's why it's used for bowel-prep before colonoscopies. 1 capsul of MiraLax should not cause this.
If you need immediate relief, why not try a glycerin suppository?