I also recently turned 50, and although I did happen to go on a 68mi ride with friends today, back on my birthday I did 20mi of dirt on Friday and 30mi of road on Saturday. Easier than all at once.
On today's ride, I was hoping to hit 50mph going downhill (North) on the Torrey Pines Grade, but there was WAY too much headwind and I only got to 41. Probably the last chance in my life to ride my age in mph.
I'll give it another shot Aug or Sep during Santa Ana season, because there's a long and fairly steep hill near my house where I could benefit from a strong westbound tailwind.