Just do it, you'll be fine.
On a ride with 7,000 participants there will be plenty of services and there won't be a back to hang out in. There are going to be riders all around you all of the time. Ride your own pace, try to keep a little bit of space around you and don't change lanes or do anything sudden without checking out what's behind you.
__________________
My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.