I talk a lot about starting your exercise program slowly and building up your strength over time. This is a great principal, but the reality is, while you're in chemo. or radiation, building up might not be what you can really do. Depending on how your treatments affect you, maybe all you can expect is to maintain your fitness level, or even just minimize your decline. There is so much variety in treatments, and everyone reacts differently, it's impossible to make blanket statements about what to expect. All of us who have been through it know: there really is no way to predict. It's like some weird, mysterious adventure that we figure out as we go along.
I kept working out all through my treatments, but after 6 cycles of Adriamicyn (I don't know if I've spelled that correctly), what I could do in a workout was vastly different that what I could do at the start. But I kept doing as much as I could, and I tried to push myself a little, doing just a little more than what was comfortable. Yes, sometimes I would get frustrated by how little I could do, but mostly I was happy to be doing something. Exercising, even just a short walk, was something positive that I could do almost every day. My strength and stamina definitely declined, but it declined so much less than if I had done nothing. And my recovery after treatment was finished was easier; other survivors I knew were shocked at how quickly I regained my strength.
So, keep walking, biking, lifting weights, whatever you like to do. You may be able to completely maintain your strength; you may even be able to build your strength. But if you do get weaker, don't be frustrated. You are still benefiting from your exercise, you just won't see the obvious benefits that a healthy person would. Modify your exercise routine to fit how your body is reacting to treatment. Modify as much as you need to, but keep trying - it's well worth it.