I hurt my foot last night. It's not broken, just a torn ligament. Dr said it takes longer to heal but I can start running sooner (about 2 weeks). No running, walking more than necessary, swimming or cycling until then.

So how do I keep from backsliding aerobically? I was really making some progress but 2 weeks seems like it could set me way back. I can do weights and upper body workouts, but how do I do cardio? Anyone been in a similar situation?

Well, I know my gum has a hand cycle to get the heart rate going. Threes chair aerobics too, but honestly, you will still need to ease into things and you will not a backslide as you are not using your legs.

It is ok to take a break while you heal.

2 weeks will not set you back. Once you recover from your injury, you'll be able to get back all your endurance and strength in very little time. I
highly doubt you'll lose any endurance and strength at all.

Puse ups, squats, they get my heart rate up.

Two weeks isn't long at all. I wouldn't bother trying to figure out a full-out cardio workout to fill in the void, but you can supplement with compound exercises such as squats and push ups like Palestrina mentioned (though maybe "girlie" pushups so you are not putting pressure on your foot).

When I do resistance training, I circuit train which keeps my heart rate up. You don't have to be moving fast to get a cardio workout, you just have to be working hard.

Thanks for all if the ideas. I also found some great videos on YouTube that featured cardio workouts focused on the upper body.