Gazelle modification?

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Ok, so YEARS ago, I bought a Gazelle (the one Tony Little promoted). I was a broke college kid and needed something to do at home when I couldn't get to the gym. It was in storage until last week when I decided to dig it out because now I'm a broke adult that needs something to do since I can't get to a gym most nights. LOL

I usually do walking/running weather permitting and I have a few different exercise tapes (30 day shredd, Tae Bo, etc). But I figure I can use the Gazelle to mix it up a little. When I was doing it before, I remember it's not really a challenging workout (at least not for me). Does anyone know of any ways I can get a little more out of a workout with the Gazelle?