Mike, how you feel during your fast will be a very individual thing. We are all so bio unique. For a very cleansing diet you want to utilize chloryphyll in things such as wheat grass, blue green algae (live such as in e3live) and dark leafy greens such as kale and collards. Go easy on the Sugar vegetables such as carrots and beets.
Wheatgrass is best juiced and drank fresh. Enzymes and other nutrients oxidize rapidly greatly diminsishing its power.
How is your regular diet? Depending on this you may want to try a 2 day or 3 day juice cleanse before trying a longer one.