I see two ways to handle it.
Let's call this way the standard method.
Make a time limit. Look at their speed. Make the distance too far to get there within the time limit, both along the road and through the wilderness.
You might not want to tell them how much time they have; if you don't it'll increase the tension and force them to gamble.
Then let them come up with a way to get there faster. Come up with a few rulings for what you expect will be their standard solutions.
1. Forced March. Have them make Endurance tests each time they push on. The DC should be based on the environment they are travelling through; a flat road is pretty easy, a road through rolling hills more difficult, etc. They have to make checks for each hour after the first 10. Up the DC by 2 for each hour. Failure means they lose a healing surge. When they're out of surges, they take HP damage = 1/4 their max HP total.
They can use Aid Another; if they do so, have the PC with the lowest Endurance check make one check for the whole group. Don't tell them this, only allow it if everyone is helping out, the stronger guys carrying the weaker guy's gear, carrying the weaker guy at some moments, etc.
2. Cutting Across the Wilderness. If they go off-road they might be able to pick up ground by finding game trails, ancient covered roads, or just by hacking their way through. Make it a difficult Nature check; they may also have to make Endurance checks if they are pushing on. If they succeed at the Nature check they'll find a path that saves them some time; if not, they'll lose time as they wander around in the wrong direction.
3. Magic. If it adds to their speed, recalculate how much time they have. It might be enough. If the PCs have access to a lot of this, you might want to take it into account when you initially calculate the distance.
4. Wandering Monsters. This puts their Healing Surges and Extended Rests at a premium. You can run standard difficulty encounters and they'll be challenging because the PCs can't just rest.
5. Player Choice. Add in a few branches that the players can choose. Make one quicker, the other longer. Allow the PCs some way to get hints about which way is quicker - talking to monsters or animals, maybe, or by casting Wizard's Sight to look down from a high vantage point. Teleporting up into the air might work too!
The Skill Challenge method:
Make the road and the world a mean, spiteful character that wants to stop them. Anthropomorphize it. It has desires and goals and character.
In this case, it wants to stop the PCs using whatever means it has.
What can it do? It's your world so you'll know better than I, but here is what I see:
- It bends and twists over hills and roads, making the journey long and taxing.
- The road is treacherous, roots and loose stone always threatening to twist an ankle or cause someone to trip and buise a knee.
- It calls forth monsters along its path.
- It can influence the weather, starting with the heat of the blazing sun. After the heat dissipates, an electrical storm blows in. Sheets of rain slow progress and block sight.
- It washes out bridges and drowns fords.
When you run your skill challenge, describe the environment and ask the players how the PCs are going to deal with this threat. Each failure will cost the PCs time and possibly other consequences.
