The relative level of severity in any training session is entirely dependent on the level of the participant.
We have to "pace" ourselves, but we should also be putting ourselves "on the edge" often. It's your teacher's job to adjust this in ways that the student sometimes cannot do for themselves (for obvious reasons).
Seminars are great for providing the venue for new experiences with different teachers, partners, styles of practice, etc.
I travel and teach seminars frequently. I try to have my seminars be balanced between learning new things and pushing yourself to the edge of your envelope with things you are familiar with. The seminars I do are mostly with members of Jiyushinkai, so I am also involved in an ongoing relationship with most all of the students.
I recommend going to seminars as often as possible. When you're relatively inexperienced, ask your teacher for their view about which seminars are best for you. Once you have reached the ikkyu/shodan and above range, then travelling and doing "shugyo" is very good training.