Bringing a horse to a high level of fitness takes a long time because you should always start a horse out going easy and increase the time and demands as they will let you know when it is ok to step up the demands. Patience will play a very large part in this process. Pushing too hard, too fast is asking for trouble with muscle soreness and inevitable joint issues. If your horse starts to lather down, this is a big red flag. Either you are pushing your horse too hard or they are experiencing pain. There should never be lather on your horse; a good strong sweat but not lather. Have a training schedule in mind and try to stick to it and remember that you cannot get a horse fit by riding them once or twice a week for ten or fifteen minutes. You must have a safe and consistent plan, riding every day or at least five or six days a week. So my suggestion is to be kind but be stern and before you know it, you will have a fit horse that will enjoy their job and look like a picture of health. This is taken from this website. Determining the Fitness Level of Your Horse by Appearance & Touch, and Recognizing Sweat Patterns. PREPARATION AND CONDITIONING
my one TB is a highly competitive field hunter. he lathers in nearly all weather, and sweats profusely. always has. he's also regularly seen by a vet (who btw is also our hunt teammate) as well as chiro, and he's more fit now than he has ever been in his life, at 14 years old and topping the scales around 1400+ pounds. pretty muscled for a 16.2h TB....
he lathers every ride that is 30 min+. always.
lather is not necessarily an indication of fitness or conditioning, but can be tied to genetics. some animals sweat more than others, some don't sweat at all. some sweat easily. my horse is like me - we can hunt in 55 degree weather and by the end of the course nearly 2h later we're both lathered. but the next day neither of us is lame or injured.
in addition, in peak hunt season, my horse is ridden only once - twice per week - either one - two days before the hunt and the day of. he gets all day turnout and exercises himself in TO and maintains fitness. similar to racehorses that get galloped just once a week and a month off between races. to GET to peak fitness he's in work 3-4d/week starting in 20-30 min increments, building up to 2h - 3h on trails at varying mpm speeds (meter per minute).
conditioning a horse is as individual as that horse. my clyde cross turns to mush if you don't work him 3d a week and he'll never be able to hunt at the pace and speed of my TB. they are very different animals.
my point is: lathering is absolutely NOT indicative of conditioning level. heart rate, resp rate, and return to resting rates are. just ask any endurance racer. to GET to peak conditioning, the methodology will also vary by horse. what will not vary is being able to judge how to push a horse into their fatigue zone (tired but still uninjured) which builds fitness and NOT into their failure zone (risk of injury and undue stress).