trotting hills is an awesome workout. i do it 4 times a week before or after flatwork. lunging is great too. i lunge all my horses 15 mins before riding. and make sure to ride your horse correctly on the bit, lifting the back. we also have a water gym and a walker for our horses which is extremly effective but not every barn offers it.
To get a horse back into shape you need to be patient and take the resumption slowly. Moreover you'll need to make short sessions. Schedule the sessions over a month or more is very important.
I would begin the first week with short sessions of lunge of 20 min maximum mostly trotting and walking.
The second week you can begin to slowly work on the flat.
The third week i would recommend short trotting of 30 min sharing you time between walking time and trotting time without tiring your horse too much.
On the fourth week when you're horse will be into better shape you can begin to jump small fences and make longer trotting and sessions in general.
On the fifth week you horse should be back at normal and be able to withstand the usual training. Of course a lot of grooming and turn your horse out to the pasture can help the morale of your animal thus help the recovery.
Doing a medical chek-up during this period and at the end of it is highly recommended.
Trot sets! Put your horse on a lunge line and if she/he is being stubborn don't be afraid to use a whip. (I usually just kind of trail it behind to keep the horse going) Trotting builds the most stamina out of the gates (minus galloping). Start with 2min trots in 3 sets(walk,trot,walk,trot,walk,trot and walk) and after those 2 mins walk 3 mins in between. After a few weeks your horse should be noticeably starting to get back into shape.