Hi Christine
China will be cold and dry in March - Beijing, for example, will be between freezing and about 10 degrees Celsius, so I would pack plenty of warm clothes.
http://worldweather.wmo.int/
As for walking, you won't have to do too much if you don't want to - there's plenty to see even if you don't wander far. There may some steps along the way - at the Great Wall in particular - so it would be a shame not to make it up those. If you don't already have one, a stick to ease the pressure on your knees would be a big help here.
I'm sure you will have a fantastic time.
Kind regards
Olly, your editor