Recreational riding is about going places.

You could shorten your distance to zero by not leaving the house at all, but that's missing the point. I like to go places, and the further I can go on the same amount of energy the better.

1) You may "go places" by driving your car (that goes 50mph) closer to your target place. Then use your "unoptimized" bike to reach your target at shorter distance.

2) "shorten your distance to zero" is too extreme, because we need some physical exercise and want some enjoyable experience.

How exactly do you shorten trails, or race courses?

> How exactly do you shorten trails

You do not go all the way in.

Or pick shorter trails.

Or allow more time.

> race courses?

You may do less lapses.

Or pick a different (shorter) race.