We live in quite a hilly area that is also bad for my asthma, and I often find my legs ache badly when I'm walking uphill. (It's the feeling you get when you exercise and push yourself a bit further than normal only it happens on normal walks)
We've just come back from a week's holiday where I didn't get any asthma symptoms, and I was doing some really steep climbs up hills, no problem. Since getting back, the asthma's back to normal, and this morning, my legs were really aching by the time I'd walked uphill for about 5 minutes.
It's obviously not lack of fitness, because I was walking much further, over more difficult and steeper terrain on holiday, so I wondered if it could be connected to the asthma? It does seem to be worse on the days when my breathing is playing up.