Dog with a death wish!

[Blueskytoday]

We have an 18 month old cavalier kc , in the house she is the sweetest dog, very affectionate, calm, well behaved. I chose this breed expecting this kind of laid back, calm behaviour. She is very clever, learns commands etc very quickly.
As soon as we go outside she is like another dog, constantly escaped from last house to chase chickens nearby, has chased sheep and lambs, has run onto road. I have been really shocked at how fast she is and how high prey drive seems to be. I'm scared she is going to get shot by farmer or run over etc. Im not letting her off lead at present, some days her recall is excellent, then next day she is off like a shot. She doesn't ever seem to tire, she tends to do a huge circle then returns to me eventually , just as Im thinking she's never coming back
I have been following total recall book and using whistle, she is very food orientated and this did seem to be working well then she's run off again last night.
Any advice on how I can get her enough exercise without going off lead?

Jog with her
Invest in a tracking harness and do some scent work
Join an agility club
Join a flyball club.

In a safe area use a heafty long line attached to harness. I use one meant for a horse, you can buy long lunge reins for ponies too that are lighter. It will give her some freedom to let off energy, do dogs things and you can always find her. You really do need to find a way of making yourself the most exciting thing out there using treats, games, your voice and so on.
You do need to check your bounderies and block all her escape routes especially if your near livestock. I used anchored heavy duty wire netting to stop Hippo-pig pushing through hedges.

Take her to KC Good Citizen obeidence classes. There not silly prices and you get the support of the trainers to troubleshoot on areas your dog is struggling to get.

Agree with using a long line, you can let it trail on the ground then step on it if dog shows signs of running off, sniffing the air etc.
After an embarrassing incident I've started using one for my greedy lab in places where there might be people having picnics . It seems to be working, and I'm hoping I can train him out of scavenging dead rats etc as well.

Thanks for all the ideas, will get long line
It's amazing that such a cute little dog can get in to such trouble, I thought I'd bought a lap dog who would be happy with a quick walk
I definitely need to get fitter so jogging with her or maybe power walking will be a help, even though it might be [embarrased]
Ender the dead rats is

She needs brain stimulation as much as exercise. If you exercise a dog like this too much, they just get fitter and you need to exercise them more. Work her brain and she truly will get tired.

In your shoes I'd use a technique called NILIF - even after a couple of days dogs will often be paying their owner much more attention.