I've started doing pretty long warm ups for sprints now. Funny story last year about mid season I was drive to a race about 1hr from my house. I'm about 10mi away when my car broke down so I call for a tow its going to be awhile this is an hour before the race start. My gf was with me and suggested that I just ride out to the race rather than waste the race fee and she would wait for her folks that where bring out her car (great people). I when for it and and started hammering a way to get there on time then bumped in to a local rodie that was going to the same race to watch and offered to pace if I told him where to go (he was lost). Did the fastest 10mi of my life got there with about 15mins to spare with all my wetsuit, shoes, bottles even a bin for water to wash my feet in all in a backpack. And you know what it was a PR race and I felt great the whole race.

Bottom line: Shorter the race longer the warm up. Make sure to include some high intensity intervals as well in there too.