A lot of people believe that if you're sweating, it means you're doing hard work. That's not the case. Indeed, if you're sweating, it means the work is hard to do. However, if you're forcing yourself to sweat, by wearing a sweat suit, then it just means you're losing water.
In any case, doing 100 crunch or 100 situp is really nothing. It's a walk in the park for your body. If you're having trouble running, I recommend doing other exercises. Most people overlook walking, but you can be surprise by how much you lose by simply making time to walk 30 minutes for a fast walk around the block.
I find that a lot of people have confidence issues when it comes to walking or running outdoors. A lot of people will say, "Won't people laugh and stare at me?" Unfortunately, there are a lot of bad apples out there who will attempt to make a joke about you. However, at least you're trying. No one can joke about someone who is out there trying.
So just go outside, do something. Don't force yourself to sweat and possibly have a stroke. Just go outside, do something for 30 minutes a day. And you'll be fine. It doesn't have to be a hardcore, sweat till you die, 1000 crunches or nothing. I mean even Navy SEALs don't train like that, despite the reputation they give. Just play it smart, and be efficient while being effective. Just take it one moment at a time. Don't think about the future or how you will be like in the future. Just concern yourself with now, and you'll be fine.