Congratulations. I still remember my first century. In spite of certain sensitive areas being absolutely numb for four days I was addicted for life. Nearly four decades later I still remember the nice people I rode with that day (and the Brooks saddle that never did get comfortable). So, the obvious question: when's your next one?
As far as the tandem, VaultGuru is right. 60+ mph is no biggee. My wife is usually captain but I control the drag brake. Well, sort of; when she tells me to brake, I brake.