My cardio fitness isn't very high atm (I'm not getting out kayaking at all and no other exercise). Just started going to gym twice a week and they have a new machine, hand-pedal machine.
Sit down and pedal with your hands - you can set goals such as an rpm for a set time, an effort level etc. Favourite so far is rpm for set period - it varies the effort level so that my required wattage goes from 50 up to 300 to maintain the rpm (I choose 90). 300W from arms-only kills me, I can only sustain it for a 30s or so. 100W is sustainable.