I wonder, Mimi, if the work you're doing on a Tandem is all the same aerobically but on a slightly lesser scale strength-wise, so while the long rides on the Tandem have done great stuff toning and improving your endurance, a shorter ride on a roadie (at the same speed you'd expect on a Tandem) has your muscles screaming "HEY! LUNGS! HEART! WAIT UP!!!!!" Just a thought. It's early and I haven't had my coffee.