I’ve really got into using the Concept 2 recently doing 60 mins on level 8 at av. 28 strokes per minute and I find that the weight i have been trying to lose for ages has dropped off.
I find that the first and last 10 minutes really hurt, but the main thing is too keep good technique and form, don’t stand start to use short strokes or stop using your lower back and legs when you get tired.
it is very repetitive, but you can use this to your advantage, in that it allows you to “get in the zone” and switch off, the main thing for me is making sure that my breathing is regular and that I have good music that suits the tempo well (for me Prodigy, Queens of the Stone Age, White Stripes etc).
If you have good form and learn to breath well, then just gradually increase your distances.