First thing you've only got 3 - 4weeks so to it does not give you much time to enjoy China if your travelling.
Only if you really have to would I go to Xian this trip.
You can get some good discounts (70%) on 5 Star hotels on sites such as http://www.asiarooms.com/china/shanghai.html?gclid=CNuKvN3bvJUCFQlStAodwHmQYQ
Good Train travel site http://www.seat61.com/China.htm
Hangzhou well worth the vist, you can find some good less expensive restuarants on the side streets.
Also good place to get away from the hustle and bustle for a few days is Putuoshan
a buddhist island.http://www.hpmuseum.org/china/trip4/putuoshan.htm
It may be touristy but no traffic.
Also you can eat in the monks kitchens at the top of the mountain
You said you was fit so you should make up the steps.
However dont forget to buy knee pads if you intend kissing every step
You get to it via Ningbo
Nice 5 star Hotel there but you may get a knock on your room door late at nite from
female Chinese lady
if you stay in business class - be warned !