Lupus sounds like it could work for what you want. It has a lot of symptoms that could cause limping and heavy breathing, like muscle soreness, joint damage, and respiratory issues. As long as it's managed properly it's likely not to be fatal, but if the symptoms flare up and aren't treated, it can lead to kidney failure and cardiac arrest even in younger people. And if you have lupus, you're more susceptible to things like blood clots, which can cause all kinds of complications or death if there blood clot is in a bad spot.
But also, between flare-ups and when properly medicated, the person can live a mostly normal life. So if you need your character to still be able to go out and do things, they can.