I know when I am on IV's my docs still want me to do as much as I can. I remember last time I was on them, my home IV nurse told me not to use the tredmill because it could make it easier for my midline to clot off - when I told my nurse practitioner at my CF clinic this, she nearly flipped out! She told me "You tell them you HAVE to exercise for your lungs, especially when you are sick". They also have definitely said not to overdo it, though, because I need to give myself enough rest so that my body can keep fighting the infection. They just want to make sure I am doing all the airway clearance that I can.
So I try to do as much as I can handle, without pushing it.
To answer the question about motivation...two main things help me: My dog and my husband. My dog helps because I know I have to walk her (its like I don't want to let her down more than I am worried about myself)....my husband helps because it ALWAYS helps to have a buddy or a partner to be active with. Walking around the neighborhood, working out in the gym, etc. Hope this helps you!