The pack is an ebb and flow,speeding up and slowing down. Stay near the outer edges of the pack and when the pack slows, don't use the brakes, start going around people. You'll be coasting and getting a few places.
Even when you are stuck in the middle, there is almost always a hole in front of you that you can stick your front wheel into when the pack slows.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1