One, post in the road racing foru. you'll get more help there.
Two, 50k is a bit long for a warm up, at least for me. If your focus is doiong well in the race I wouldn't ride the 50k. If you're just out for the workout and you tyypically put in long miles, it's probably ok.
For a Road race, I usually warm up about 30 minutes, 15 minutes easy, a couple 2-3 minute efforts at about 8/10ths, and one or two 30 second efforts all out.
For crits and time time trials i usually do a bit more.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.