It depends entirely on your level of conditioning, especially how much training time spent at, near, or above LT. To max out at a distance like this, it's necessary to train your body to burn and otherwise clear lactate on long rides, to hydrate and feed just right, and to be rested. Anyway, something like 90% of LT for me at your age. It helps a lot to be chasing some good riders. Can't begin to hold that solo.