Got some decent horses as certain distances but can't seem to get any new ones at other distances any advice on what distances to breed with what to get decent horses at various distances

I find the best way is to do it naturally. If you start at 5f or 6f and breed those horses i.e 5f with 5f and 6f with 6f until both horses are 7yo+ the game seems to naturally start giving you horses at the next distance up. It seems to retain the speed within your breeding, but allows you to move through the distances by adding more stamina the older the bred horses are