Like the mode of travel named in its title, "A Horseman Riding By," which begins at 9 tonight on Channel 20, sometimes moves rather slowly. It is a Victorian novel that happens to have been written in the present generation, and it was filmed in England, where they still take time, now and then, to do things right.

The novel, by R. F. Delderfield, on which the first part of the series is based, is entitled "Long Summer Day," and it shows the peaceful life of the English countryside in the years between the end of the Boer War and the first stirrings of World War I - particularly the story of Paul Craddock (Nigel Havers), who buys the large estate of Shallowford in Devon and settles in for the long job of having himself accepted by his tenants as their squire.

The slow pace and peaceful atmosphere are, of course, deceptive. "Don't have any illusions about simple country folk," a neighboring landowner warns him. "There's no such thing.Between man and his master, it's dog eat dog - by God, I think they'll eat you up."

The illusions evaporate quickly.Before the end of tomorrow night's episode, Paul's aristocratic wife is in the throes of a dangerous childbirth attended only by a roaring drunk country doctor (slightly overplayed), while Paul is off on the moors trying to track down and stop a henpecked tenant who has gone bonkers and is threatening his wife with a scythe.

Tensions are building up on all sides: Petty rivalries among tenant families; the feeling that the new squire is on probation among his inbred, provincial tenants, and also among other landowners, who suspect that he may be letting down his class; the struggle to bring back prosperity to a property devastated by its spendthrift former owners, and the approach of changing times symbolized by a primitive motorcar, the suffragette movement and a visit from a liberal political candidate.

It is all developed slowly, naturally. The story has 13 hours to be told and can dwell lovingly on small details and pause now and then while the camera sweeps across the green English countryside. It may not appeal to those who like to see cars going off cliffs, but the early episodes promise a good story well told.