Ok, this is what we used to do with my father's polo ponies coming into the season. Ours were chucked out in the field 24/7 over winter so did a lot of walking around as not stabled but if yours have been stabled you need to start walking them for the first week.
I used to do them in sets of 3, riding 1 and leading 2, they will most likely be used to this.
Week 1 - 10 minutes walking, 5 minutes trotting, 2 minutes canter (get off their backs or do rising canter), 5 minutes trotting, 10 minutes walking off.
Week 2 - As above but do an extra 2 minutes canter and 5 minutes trot.
Week 3 - Increasing canter to 2 lots of 3 minutes.
Week 4 - Increasing canter to 3 lots of 3 minutes.
Week 5 - Increase canter to 3 lots of 4 minutes.
Week 6 - Keep them at that workload.
Alternate the horse you ride so they all weight bear twice a week. If in addition to that, you can find time to stick and ball them for 5-10 minutes a day at canter by Week 4 that will up their fitness.
From an injury point of view, polo ponies are shockingly poorly warmed up for the most part. Try try try to give them a 5 minute walk before hammering them and also try and do some gentle circles with them as they do all their work in straight lines and I found they stayed in better nick if they were schooled in circles. Also make sure they remember how to neck rein (ours used to need reminding!) and when cantering them, try to push them on for 30 seconds in a fast canter, then bring them back for 30 seconds etc. They will need to play fast for 6-8 minutes in canter so need to be able to keep up. Hope this helps!
Pretty much as above, but I used to do 10 minutes canter work twice in each exercise (this was for high goal ponies though).
Also, we used to do a couple of weeks walking before introducing the canter work. Definitely worth doing some walking work if you have an oldie or two in your string.
Feel free to ask for more info - I did 8 seasons as a polo groom and got every horse through the season without significant injury.