Indoor natural body weight substitute for walking 10K 3X per week?

Hi,

I started a walking regimen 3 months ago and have worked up to being able to walk 6.6mi 3X per week at a nearby park.  It takes me 2hrs.

Unfortunately the weather is getting bad and when it gets cold I won't be able to walk outside anymore (temperatures below 55 degress trigger my asthma). 

Is there an indoor regimen that I can do using natural body weight which can substitute the exercise I get through walking?  I cannot do squats because my knees are bad.

I'm looking for something I can do in a small space so that when I travel I can exercise wherever I am without having to look for a gym.