For my first European tour I planned on riding 50 km per day and ended up averaging 65 km per day. I am in my late 50s and it think that suits me. My longest day was 89 km. I would typically leave about 9 in the morning and be done mid-afternoon, which gave me plenty of time to find accommodation.
Even though I averaged 65, I think 50 is still a good goal. It was seldom that I had a day when there was not extra distance because I got lost, had miscalculated, took a side trip, or had trouble finding suitable accommodation. Then, once settled in, my bike (or foot) was my transportation to dinner, shopping or an attraction. There were a couple of times when I found nowhere to stay in my destination town and had to ride on, once riding 97 km on my second European trip.
I like to take 1 rest day per week.