I just want to say that stationary bikes are BRILLIANT for muscle. I've been riding one for the last 3 weeks or so in an attempt to lose weight, doing 1 - 3 miles a day at around 15 mph. I'm losing fat, not weight - I have packed on the muscle so much so that the scales haven't changed, though it's easily to see the changes already. My stomach, arms and legs all ache in the good kind of way. To up the ante, I keep my arms bent, lean over the handlebars a little, and tense my stomach.
Walking is good too, but you should try power walking as the exaggerated movements of your arms will really help.