It's pretty hard to kill a Wisteria. Now would be OK to prune if you don't mind losing this year's flowers (or have you had them? - I'm in the Chilterns and mine's not showing buds yet buy in London they in bloom).
The branches are very robust - if you let them down gently from the supports, they are unlikely to break, so you don't need to go back to square one to get a good framework.
One word of caution - some wisterias are grafted and if you are too hard on the pruning, going back to the bottom two or three feet, the rootstock may dominate and all your growth will be from it.
I'd think you could keep a lot of what you've got, just shorten or cut out some bits and fasten the rest back in.