Not sure about hacking, but you are not too far form forest edge EC which has an indoor arena, Ely has a EC with a XC course, you are not too far from hunstanton beach!!! and Burnham Market (xc) or Blackwater (xc)
There's a few places around ryston end i think, i used to help out at watlington RDA group which is fab.
There's a riding club at Downham market cant remember who runs it but they have lots of events on.