You could also possibly drop off some of the training time by having the crew watch Thom's 15 minute training video at:
http://www.youtube.com/watch?v=geZUduFE4vY (this assumes that you will have internet access).
I personally am running 30 minutes missions (25 minutes + 5 minutes of crew change), with a training session before the mission starts (concurrent with the previous crew's mission, but in the ante room). This would allow for training, getting the feel, and then coming back to play another mission if they want more. This allows for a greater turn over, gets them nearly long enough to finish an Invasion, and whets their appetite for more. Additionally, once trained, they can come back without the need to train again.