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by how many gear inches (if any) do you typically drop down for the longer distance?
Not at all. You should be riding 25s at almost exactly the same speed as 10s. In fact, you don't need to gear down for 50s either, you'll be going slightly slower (unless you're really good), but you'll also want to drop cadence a bit for the longer duration. Going 1mph slower, which is normal for mortals when going from 25 to 50, will drop your cadence by about 5rpm for typical speeds and gears, which is probably about right.
Fixed wheel TTers - when comparing your gearing for 25s and 10s, by how many gear inches (if any) do you typically drop down for the longer distance? Ta.