My answers to your last couple of questions: Yes, absolutely push really hard in the beginning to stay in the leading group, and it is likely to be a bunch of 400-500W efforts. When you have to close a gap, it is more of a power effort than an all out sprint most of the time. But if you are in doubt of getting the gap closed, go with the all out sprint. You'd be surprised at how often you think you are spending your last effort to get back on the pack, to find the pace lull for just long enough for you to recover and stay in the race.