the two address two different types of fitness.
interval training, which is what a trot set is, conditions cardiovascular strength and rapid respiratory recovery. the duration of intensity coupled with the appropriate duration of recovery should, after a few short weeks, yield a sharp increase in cardiovascular recovery after intense work. trot sets are enormously expensive from a physical standpoint when it comes to how much they task the cardiovascular & respiratory system, so should only be done in short sessions (IE 2m trot, 1m walk, repeat 5x) and only done a few times a week. unless you are conditioning for UL eventing (prelim+) there should be no reason for each ‘set’ of effort to exceed 5 minutes. interval training is great for sports that require extreme but short bursts of effort like eventing or barrel racing, but if you need something that requires staying power and endurance like hunting or endurance, you need to condition by hacks and roads.
interval training is something that is low-intensity on the limbs, so it is good when you have a horse that is not in the best physical shape that you want to ‘leg up’ for an event; however, a horse pulled from a field should not be doing trot sets until at minimum they are conditioned enough for a light ride in the ring.
interval training, if done on hills or out in the field, can help condition the topline and produce a “5th leg”, however, the short spurts of intense exercise followed by brief let-down is not going to condition the topline the same way as hacking.
hacking conditions the entire body but most importantly it conditions the tendons and ligaments. in the long format days it was not uncommon to combine hacking with “loading”, which is done to strengthen tendons and ligaments in the lower legs. hacking is much better for the horse to do over a period of time, and is considered “long slow distance” which is the best type of exercise to produce a horse that has a job that requires it to cover long distances at a middling speed. for prelim, you need to incorporate trot sets, not just hack.
hacking when done right, at a march and on terrain, will produce a very good topline.
if you are moving up to prelim you already have a baseline of fitness. one day a week should be an hour of walk hack only. 2 days a week should be dressage schooling (with a 15/20m hack before/after). 2 days should be interval training: for when i was preparing my guy to move up, we did 5m trot sets, 2m walk, repeated 3x on day one (tuesday) and on day 3(thursday) would do canter sets, 4m gallop/canter at ~400-475mpm (i played around a lot with adjusting strides and speed in my sets due to terrain) 2.5m walk. the rest of week can be for jumping. some people do not include a day off for their prelim+ horses. i think it should be dependent on the horse. some horses get a lot out of a day off, others need to be kept moving and an easy hack is a good substitute.
this has a good guideline but i’d argue it’s a little excessive for BN/N:
http://www.focusedriding.com/files/CONDITIO_bn.pdf
you will not get the cardiovascular fitness required from prelim efforts by hacking, so you do need to incorporate interval/trot sets into your weekly itinerary.
this time of year the ground can be very hard and unforgiving. a few times i have been known to do trot-sets in the ring instead of afield. you do want to condition/load the limbs because they will be going on hard ground at events, but it is a very delicate balance for sure.