If it's too hot to walk outside, then go to your local mall and walk inside. Lots of people mall walk before the mall opens. Walking inside will at least give you access to a/c.
If you have problems standing for long periods because of your knees, then do seated exercises. Coach Nicole has a couple of cardio workouts that can been done while sitting in a chair. Work your upper body muscles if you can't work your lower ones. www.sparkpeople.com/resource/videos-detail
You can also search on YOUTUBE for more chair exercises or chair workouts. Some of those workouts can be quite vigorous. So, do what you can to start. Your strength and endurance will increase with time.
Do you have a set of resistance bands ? If not, you can find them for cheap at any Target, Walmart, Sears or even TJ Maxx. Do some upper body strength exercises with the bands. working with resistance bands is another way to workout at home. Strength training should be a part of your workouts. You don't have to do a lot to start.
Do what you can even if it's only five minutes. You've got to start somewhere.