How much and how frequently should one walk during the third trimester?

I walked more during my 2nd trimester. When I am in my third trimester now, I feel breathless easily and whenever I walk too much I will have cramps and need to bedrest for few hours to ease the pain. For my first 2 pregnancies I definitely walked a lot during 3rd trimester as I was still working that time. I feel my age and body is catching up and unable to cope with walking too much. Thus, I try to reduce walking for this pregnancy. As long as you feel comfortable, you can walk as much as you want but do remember to have a short break in between in order not to overtire yourself.