I use a rowing machine. When I'm relatively untrained, I start with rowing at a steady rate, working up to 20 minutes. Then I add in HIIT, starting at 20 seconds work/ 40 seconds recovery. When I can do 6-8 intervals, I'll change the rest/work ratio to 30/30, then 20/10. For the work intervals, I row as hard as I can, a pace of around 1:30-1:40 per 500m. For recovery, I row as slow as I can, around 4:00/500m.
