The gap between two fences, measured in canter strides: three strides is roughly 14–15 m, four 17.5–18.5 m, five 21–22 m, six 24.5–25.5 m.
\"Riding the distance\" means choosing a pace that lets your horse fit the planned number of canter strides in cleanly — neither crammed nor stretched. That's why experienced riders walk the course first and count double paces, since one double pace is roughly a metre — calibrate your own pace against a tape measure once, or you're counting someone else's metres. For ponies, subtract half a metre to a metre per distance, because the canter stride is shorter. Combinations are the special case: elements sit only one or two strides apart, you have virtually no time to correct, so the pace has to be right before the first element. And the safety rule behind all of it: you fix a misread distance with a gentle change of pace several strides out — never with frantic driving just before take-off.