It’s great to hear that you all are all set and raring to go, and there can’t be better way to prepare, train yourself than hiking as that’s what you will be doing bulk of the time in trek. Most of the treks in Bhutan are high altitude ones, except winter trek in lowland region. So physical training in any form would stand in good stead in terms of enjoyment of trip.
For general information purpose in regards to your preparation, Please visit the link below.