As long as you can make riding a priority and do the training for the next 12 weeks and stick to the schedule, you will be able to complete the 129 mile ride. You may not set any world records for speed, but completion is the goal, right?
If you are not able to ride that often and keep coming up with excuses over the next 3 months, well, then, there is a good chance the 129 miles may be a bit much.
I rode my first MS150 last year and stuck to a century training schedule pretty rigorously, and we rode 100 miles the first day and 75 the second and it was not nearly as hard as I thought it would be because I had trained so well for it.