Whooooooooooooaaaaaaa there hun.
You need to condition the horse first! How long has she been off??
If it's been for more than 4 weeks, I wouldn't be even thinking of cantering right now. If it's been more than 2-3 months I would only be walking for a while.
And in any situation with a horse that's been off, you need to start your conditioning on a good, flat and fairly soft surface (aka an arena) to avoid undue strain.
Talk to your vet about a good conditioning program to put your horse on.