Abu Daoud at Islam and Christianity has asked a very good question. Well actually, he states a fear. In his post Missionary Secrets 1 -- Retirement worries us, Abu discusses how "retirement" in the [recent] Western tradition looks for missionaries.
Honestly, I haven't had too much contact with "missionaries" because, well, they're "over there" most of the time, and when they're home on furlough, it's expected that they will be fundraising in order to go back. They don't seem to have much rest. I once attended a church that suddenly had an elderly couple show up and attend regularly - a couple that were life long missionaries who had returned from the mission field. For me it raised a whole new set of questions about missions. Do you know missionaries that have returned? Were they treated well by their brothers and sisters?