If you can hang with them relatively easy on a 65 mile ride you should be fine on a 200 as long as you have a good base. The key is to not get in the red zone or you will be in trouble. In the next 6 weeks I'd just be working on speed (with a few longer rides thrown in). As and example, if you are at comfortable with a 20mph pace for 65 miles a 15-17mph pace for 200 miles is much more doable because you never push yourself anywhere near your limit. The longer or harder you push your limit the longer it will take you to recover. Your recovery speed may be 10-13mph which will really slow your average down. The key to fast long distance riding finding an output level that you can maintain over a longer distance. The faster you are used to riding the faster your long distance output will be. When I train for RAAM or a 1200k brevet most my training rides are usually in the 40-50 mile range (working on speed climbing or intervals) with some longer rides on the weekends.