The principles of excercise are relatively the same across disciplines: you increase the effort required to perform an action and your body adapts physiologically. The difference between run, swim, bike, paddling and gym sets are largely logistical and focus on different patterns, but the principles and even sometimes the sets are the same. Here is one such example.
You start with an easy warm-up effort, limbering up and preparing your body for exertion. 10-15 minutes is usually adequate, but you can also get away with 5 minutes (if you’re really cramped for time). Alternatively, use part of the main set as your warm-up if you’re really stretched for time. Here goes the main set.
2 minutes of medium/hard effort, 1 minute of rest.
3 minutes of medium/hard effort, 1 minute rest.
and so on, building up to 5 minutes. 2, 3, 4 and 5 minutes ON, with a minute OFF in between.
You can do this while running, paddling or even on the ergo.
And then, you work your way down again. 5, 4, 3, 2. That’s 28 minutes of hard/medium effort with 6 minutes of recovery, plus time for warm-up and you have yourself a 45 minute timed set.
Of course, you can modify the structure of this program and start at 1, end at 8, have a flat top (i.e. do 5minutes on the way up AND down) or have a pyramid top (i.e. only do one 5minute exertion to mark the middle of your set).
Rest assured, this is a relatively hard set, so your next training could be taken down a notch to allow for recovery.
Work hard, sweat and tears but above all, enjoy!