You should do fine. There's always a first time for everything. The most important factor is that you are in shape. I've been on many cross-state supported tours where lots of riders do virtually no training and still manage to ride 60+ miles per day. A few comments:

-- Lay out all of your gear and weight it. Then meticulously go over everything you plan to bring and determine which items aren't necessary and remove them. Pay particular attention to heavy and bulky items.
-- Bring clothes that you can layer for warmth. Clothes that are easy to wash and dry. Clothes that are not bulky and heavy.
-- Do you plan to bring rain gear? I would unless you plan not to ride in the rain.
-- You definitely need at least one good blinky tail light. A good LED headlight that can double as a flashlight, such as Fennix.
-- Sounds like your LBS guy knows what he's talking about. Listen to him. I agree about carrying three tubes and at least one spare folding tire.
-- Use the chamois cream, particularly if you are already using it.
-- A good cycling cap will keep sun and rain out of your eyes, and help control sweat and sunburn.
-- Don't forget sunscreen, pain-killers (eg, ibuprofin).
-- Wear cycling shoes you can walk in, such as mountain biking shoes with cleats.
-- Get at least two pairs of good cycling shorts (preferably bibs) so you can wash and dry the pair you aren't wearing. A small bottle of Woolite is great for washing clothes in sinks.
-- You're right to ditch the trailer. That would just encourage you to overpack.
-- Don't skip the rest days. Your body will need it. Also take some recovery days where you ride easier, particularly after long and hard days.