I work all day, so I always make mine ahead of time. It tastes better if you make it fresh, and I've read on the forum that the lemon juice loses its nutritional value the longer that it sits (which makes sense)...but I have to do it this way. It seems to work out ok for me.
I usually mix up the lemon juice and maple syrup - making a concentrate - which I take with me and add water as I go. That way I'm not lugging around 60+ ounces of lemonade! :)
Don't put the cayenne in until just before you drink it. The pepper will "bloom" as it sits in the liquid and continually get hotter throughout the day.
Good luck on your outing. I remember my first MC when I started telling people what I was doing, they looked at me like I was looney! So I can understand why you don't want people to know.