Actually, doing 55mph should not shorten the life. If you go to
http://home.comcast.net/~pat1776/Alaska05/index.html (copy and paste into your browser), you can read the story of a guy who did 10,000 mile Alaskan road trip on a gz250 in 19 days. He still has the bike and now has close to 50,000 miles on the bike.