I think it helps but you do need to build up fitness really slowly and ensure you do not strain them by doing too much, little and often is best, give time to recover if they do start to struggle for some reason, I would try to ride when it is cool as early as possible in the morning rather than in the evening when they have been exposed to the heat and pollen all day, lots of gentle hacking would be ideal, see how it goes, many horses with mild symptoms lead perfectly normal lives, once they are very fit it can be easier to manage.
