About elliptical workouts

"i am 60 yrs old. regularly doing weight training for the past 30 years. my regulars are half squat (80 kg), bench press (50 kg)and shoulder press (40 kg).
but no cardios. my doc advised to include cardios. I bought a elliptical. in the beginning it was difficult to do even for 1 minutes. but after 1 month able to do for 20 min continuously but with minimum speed and minimum tension.
my question is, should I increase speed or increase tension or duration of the workout?"