Is, not isn't! Hill work is great :)
I like to start my horses back up after a long break with lots of short walk/trot sessions- half an hour 3-4x a week or so, slowly making the periods of trotting longer every week. Once the horse is using himself (powering off the hind end, stretching down, rounding his back) properly at the trot, introduce the canter again in small increments. Also, once your horse is reasonably fit and balanced at the trot, hill work isn't an excellent way to build up the hindquarter strength. Just as a side note, I personally would advocate for ground driving over lunging, but they both work!
(This is assuming your horse has been off for months and is quite out of shape- feel free to skip the baby beginner steps if that's not the case!)
Hi Maren !
After a break, your horse must star activity again gradually to avoid myositis and muscle troubles. You can star step by step with strenght exercices as walking in the water, longe work, stretching ...