Originally Posted by
BlazingPedals
Work up to a bit longer rides. I'd say if you can do 75 or 80 miles, you can do 100.
Yep. It is as simple as this.
60-75 miles/week is not enough. OP needs to ride more.
A month isn't a lot of time to build that kind of endurance. He can do it but it isn't going to be the most comfortable thing he has ever done.