To help my horses become more comfortable in the contact and learn how to soften to pressure, I like to lunge them with lunging gear in the round yard. I prefer to do this training process in a halter and am just looking for them to be able to follow the pressure down and out. This encourages them to stretch across their top line, build muscle and promote relaxation.
When lunging, ideally stay at walk and trot (unless the horse is very strong in their body, they are on a big circle and on a good non-slip surface) with the aim to lunge for about 2-4 minutes on each rein until the horse is able to find suppleness in their work. For fitness training, I would lunge them for only 4-7 minutes on each rein as tight circle work is very hard on their body and joints.