It took me a couple of times reading your original post but I think I understand the situation now. By "first tour" I was assuming you were packing up camping gear, loading up your bike and heading out on a trek. I couldn't figure out why you would only be going 15K for a "tour". It doesn't sound like that is what you are going to be doing. Is this an organized ride? (These are commonly called "Charity" rides since they usually are done in support of some charitable cause. Typically you pay an entry fee and ride your distance. You get a T-Shirt and some other junk/swag) If so I would say go for the 35K.
On every one of these type rides I have done there has been a rest area at about each 10 mile interval. You can stop and take a break, have some type of snack (bananas, oranges, cookies seem to be the common fare) get water/sports drinks, refill your water bottle, hit the porta-potty. At the finish line there will be more snack/refreshments.
A 35K ride is just a bit over 21 miles. So, you would probably do about 10 miles, hit a rest area, do the rest of your distance. Since you have done essentially half your total distance already, and you have a couple of weeks left to "train", you should have no trouble with the 35K. As has been said before, don't go too hard at first. The start of these things tends to be a *bit* chaotic--lots of people in a small space all trying to head out at once. Keep your head on a swivel, (keep looking around to see who is about to hit you) and try to give yourself space as best you can. It wouldn't hurt to start out near or at the back of the herd to avoid the biggest mass of folks. You can always start working your way past slower traffic once things thin out a bit.