You MAY be lucky.
If you had snapped the cable then you wouldn't be able to get any gears and the throw would not just be mushy - there would be no resistance at all!
It's possible that you could have over-stressed the adjustment mechanism, which is incorporated into the gearbox end of the cable, and may now be slipping.
Remove your top intake pipe and move your coolant reservoir out of the way, then you can see the cable ends on the top of the gearbox, and following them backwards, a bracket where the cable shroud ends are located.
The offending cable is likely to be the lower one - this controls fore/aft movement. If the big E clip which secures the cable to the bracket is missing or there is no plastic there for it to grip (compare it to the end of the other cable), then you'll know that it's broken.
Otherwise, get someone to sit in the car and move the gear lever backwards and forwards. If the centre bit moves in and out of the shroud, then it has simply come loose, and you might be able to lock it back in place by turning the knurled part behind the bracket.
(All of the above will become an awful lot clearer once you can see it!)
You can replace one cable at a cost of about £150 inc VAT & P&P for an OEM one. But you have to remove both to do the job, and might damage the other in the process - the plastic hardens and becomes brittle over time.
The alternative is that by spending less than £100 more you could get the vastly superior AB Motorsport set via WillB, which apparently make it feel like no cables are involved at all!
As for short throw kits, IsuzuPerformance.com used to do some which replace the original selector box, but they are long since discontinued.
An alternative may be to fit an extension to the arm below the gear lever.
This was recently discussed in
this thread (which also includes drawings of all I described above).
