OK it's dawn and the group is getting ready to leave. Bags are being packed and the party leader is getting people into marching order. Time to see how far they really get today. Consider the road ahead. For each major negative condition in the next stretch, pick up a d4. If the condition is really serious, pick up a d6. Typical negative conditions are things like steep hills and slopes, muddy trails or roads, rain, exceptional heat or cold, or obstacles like rivers or ravines. Serious conditions would be extremes of the above, a blizzard with sub-zero temperatures, burning desert, mountainous terrain, or dense undergrowth coupled with ravines, streams and rivers. Let's say we picked up three d4s. The road ahead runs through hilly terrain and it's rained for the last few days so the trail is muddy and slick. There's also a river to cross, and the ford is probably flooded. Roll the dice and add them up. That's the base penalty in miles suffered by the travelers. Let's assume we roll a total of seven.
Now that you know how hard the trips going to be, you can adjust your result using your party's skills to adjust. No need to go into too much detail here, the expert teamster negates some of the penalty by keeping the pack animals moving. The ranger scouts the ford and finds an easier way through the swift waters. What's important here is to give the players that have invested in the often under-used skills like survival a chance to shine, even if it's briefly. This is also a good place to reward the group as a whole if they're careful about travel planning and preparation (and if they care, some groups don't). Apply each party skill as a one or two point negation against the previously rolled penalty. In our example the teamster's efforts keep the animals moving but the party members are still slipping and sliding in the slick conditions, so that's worth one point. The ranger's alternate path across the flooded stream is a good find, so that's worth two, reducing the party's total travel penalty to four.
So now you know how far the party traveled, 16 miles (20 base - 4 adjusted penalty). Fill them in on the day's travel. Be sure you mention the efforts of the party members that kept things moving. No need to go into too much detail, because repeated travel days can get kind of boring, but the occasional highlight is worth narrating to give a sense of the locale and to provide a feeling of movement.
That may sound like a lot of work, but it's not too bad once you get used to doing it every day. I usually combine it with an encounter check to see if something really interesting happens during the day. Some people may have difficulty coming up with negative conditions that will affect travel. If that's the case, head to the library and check out the magazine rack for outdoors-oriented magazines. Flip through a few and jot down a few notes based on the pictures you see. Remember modern adventurers have access to modern gear, which can mitigate some of the problems encountered in outdoor travel, but the less advanced adventurer is going to suffer quite a bit more from extremes of temperature or terrain.
To sum up:
- Figure the base rate of travel for the group
- Each day pick one d4 for each negative condition, one d6 for each major negative condition
- Roll the travel penalty
- Adjust the penalty by applicable party skills and preparation
- Narrate the result.