How far did Harold's army march between its two 1066 battles?

About 400 kilometres, from York to the Sussex coast, in roughly a fortnight - having already marched north at similar speed to fight Harald Hardrada at Stamford Bridge on 25 September. Harold's army fought a full battle, marched the length of England, and fought another on 14 October.

It is one of the reasons the outcome was close rather than inevitable. The core of his force was the housecarls, professional troops who could keep that pace; a large part of the militia could not, and Harold arrived at Hastings without his full strength and gave battle immediately rather than waiting. Whether he should have waited is the standard counterfactual, and the argument for haste is that William was devastating the countryside of Harold's own earldom to force exactly that response.

The sequence itself was luck of a sort. William's fleet had been held in port for weeks by contrary winds - the same winds that let Hardrada sail. If they had arrived together rather than three weeks apart, Harold would have faced two invasions at once and almost certainly lost sooner; instead he faced them consecutively and lost the second by a margin small enough that historians still pick over it.