Hangzhou is the single best-matched excursion on this list for a runner who has just finished a marathon. High-speed trains from Shanghai Hongqiao reach Hangzhou in around 45 minutes, running frequently through the day, and the city's main attraction, West Lake, is built for slow movement rather than strenuous sightseeing.
Two nights is enough to see the lake properly without rushing, and December's cooler, clearer air over the water is a genuine improvement on Hangzhou's hazier summer months.
West Lake
West Lake is best treated as a place to move around slowly rather than a single site to visit. A short taxi ride to the northern or southern shore, followed by a lakeside walk of whatever length feels manageable, covers the essentials: causeways, pagodas and pavilions viewed from paths that are flat and well maintained throughout. Boat rides across the lake are a low-effort alternative to walking the full perimeter.
Lingyin Temple
Lingyin Temple, west of the lake, is one of China's most significant Buddhist temples, set among wooded hills with a series of grottoes and carved Buddhist figures nearby. There is a reasonable amount of walking and some stairs, so this is better suited to the second day, once legs have had a full night to recover, than the day of arrival.
Lakeside tea houses
Hangzhou is the historic centre of Longjing (Dragon Well) tea production, and lakeside tea houses around West Lake offer an easy, seated way to spend an afternoon. This is as close to enforced rest as sightseeing gets, and worth building into the itinerary deliberately rather than treating it as a filler activity.
Getting back
High-speed trains from Hangzhou East station back to Shanghai Hongqiao run frequently until late evening. Buy the return ticket a day ahead during busy periods, as the route is popular with domestic travellers as well.