Most games just apply a modifier based on terrain type (1/2 movement in forests, 1/4 movement in mountains, etc.) with modifiers often stacking. A good mountain pass will greatly reduce the effort required to travel through mountains and good roads effectively allow for full-rate travel on all but the worst terrain. Similarly, a bridge over a river negates the penalties associated with such, and a ford can be almost as good. Roads may travel a long distance out of the way to to get to low-cost travel options.

One of the problems of PCs in high-magic games is that they will often have things at their disposal that negate severe barrier penalties (the D&D Fly spell is the worst one in my experience).

An important thing to remember about overland travel is that season and weather are critically important for travel rates. Springtime thaws, rains, and floods can make any but the most well-maintained roads virtually impassable. Winter snows can block mountain passes for all but a few months out of the year. Huge herds of ruminants on open plains can block travels for days here and there. For worlds with the classic evil humanoid monsters, whole areas may be virtually impassable due to patrols and bad attitudes (regular humans can work just as well here).

A critical idea is that any choke point on trade routes is likely to have a number of greedy fellows squatting on them. Many of these choke points are likely to have cities based on trade in them and the owners of those cities are often willing to help the PCs travel faster by relieving them of all that heavy coin that they're carrying. Policies vary from place to place and a a city (or merely an unscrupulous vendor) may sell tokens that purport to offer free passage through the realm but really just mark the party as easily exploitable.

There are always a huge number of options to slow down travel. An amusing one is providing the party with a nice camp site that has a pleasant cave or other shelter. One party member finds a trinket and keeps it. That trinket belongs to something like a leprechaun or other sprite that is greatly unamused, but limits its revenge to little things like quick-fraying saddle attachments, damaging tether lines to let the horses get away, adding unpleasant herbs to the party's food and an endless array of little nuisances that slow things way, way down.

Horses. There's a set of slowdowns without end.

Anyhow, enough rambling.