I don't know your age, but I think 18 mph might be too fast.
Most of my rides are 75 to 100 miles, at about 13 to 14 mph.
Thursday 77 mi, Friday 82 mi. Temps from 80* to 96*
I can do that 5 days a week.
But never fast.
Try to go slow the first day. The second should be ok.