I don't think you'll hurt yourself by doing a 70 mile ride when only used to 35 mile rides.
I don't think the last hour or so will be a whole lot of fun, though. Might be wise to go a little slower than usual for the first half.
People do 70 mile rides on nothing but water. I wouldn't, as I'd be miserable. If nothing else, I'd load up the jersey pockets with Powerbars or similar and gnaw on one every hour or so.
105 miles is a longish ride for most folks. 105 miles on the FG is a long ride for almost everyone. I'd do 70 first and see how it feels.
Have fun!