Consider keeping up the walking exercise. I'm an asthmatic, been on medication for it for over 30 years - ok, I know that asthma affects different sufferers differently, but walking shouldn't agitate your asthma anymore than a gym workout! Are you getting out of breathe because of your present weight and poor fitness? If this is the case, then walk within your abilities, and as you become fitter and lose weight, you should be able to go further and further. It does save expensive gym membership, and should be less intimidating. Don't push the exercise too far though - take care of yourself!

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Post weight-loss, now going for better fitness
Bikejoring for Fitness