my training for fast group rides and on these fast group rides, i begin in the 39 on rides and only use the larger 53 when either there is a tailwind, the road turns downhill, both tailwind & downhill, or i am getting a good draft.
on a typical 50+ mile fast ride, i'll go 39 on the first 1/3 of the ride then use the larger gears when i am fully warmed and have a good rhythm going.
i use a 11-21 cassette, so even a 53-19 is a big gear.
i also find it more comfortable to use the larger 53 when the road is hilly (rolling) so i can stand intermitantly. obviously on steeper or longer climbs, a 39 is in order.