Ok, let's see how these factors for attrition play out. You might want to keep that post open in another window to follow along with the modifiers. I am going to present this as a narrative, but I can post raw numbers for those so inclined.
Our first case will be your average peasant who wants to move some excess flour from the quaint plot of land he works on his lord's manor to market. Let's assume Forest, Cold, Fall, Trail, Sufficient provisions. He suffers no HP loss until day 5. This fits well with our implied world, a farmer would not live more than 2 days travel from a market to sell/barter his extra goods, so he can make a 4 day round trip with no harm done.
What if the same person struck out under the same conditions, but decided to not follow any trail? He would take his first hit point loss on day 4, and suffer death (to negative constitution) from accumulated damage on day 7. Does this sound reasonable? Assuming you had food and water, I believe I could survive 7 days wandering in the woods. Well, if it is a fantasy world, maybe HP loss can also abstract the fantastic dangers that lurk beyond civilization... Or maybe this is where sheltering down comes into play; parties stranded in the wilderness typically sit tight for most of their ordeal.
This time, a merchant sets out from market to move some goods a few towns away for a tidy sum. Same climate conditions, but now he is on an established dirt road. His wagon and oxen suffer no damage until day 6, or a journey of roughly 150 miles. This makes sense, 6 days of travel and 1 day of rest and no harm done. Maybe instead of traveling between markets, this is a pilgrim on a good portion of the Via Francigena (paved) heading to Rome. He can go 9 days without ill effect.
So this creates our lower boundary. For a party who treks out during the right time of year in mild conditions (total modifiers: three at 1.1, one at 1.2) can go off the beaten path for 3 days before harm occurs and if they stick to a road, this can be as long as 6 or 9.
What about sheltering down? So our party is traveling in the above climate, but they have been off any sort of trail and into deep wilderness for 6 days now. They've suffered 7 damage total (1 on day 4, 2 on day 5, 4 on day 6) over the last 3 days, but they have been able to keep up with healing. However, day 7 will inflict 6 damage to everyone, more than they can safely cope with. Instead, they hunker down in their tents to rest and heal. One day in camp will reduce attrition to 2, two days back down to 1, and four days to get it back to 0. Even just 1 day of sheltering down replenishes healing spells and gets their attrition back to a manageable level for a few more days. If they have the provisions to last and feel safe risking the potential encounters, it is a way to trek through the wilderness.
Of course, the best way to shelter down would be in a wayside inn or some other structure, which would reduce an attrition of 4 down to 1 in a day and 0 in two days.
Ok, what about a gonzo example? Parties are pretty safe while traveling during the right time of year and avoiding difficult terrain. But what happens when they need to cross an expanse of desert, or need to make a quick move during the dead of winter?
Let's take a perilous trek across Russia in the winter shall we? Forest, Cold, Winter, No Roads. First hit point loss on day 3, 4 hit points by day 5 (total of 7). Our land war in Asia is going well so far because this is a high level party that can drop tons of healing per day. Okay, 9 damage on day 6 (16 total) then 18 on day 7 (32 total) and even a high level party isn't going to be able to cope once day 8 drops a 34 hitpoint bomb.
Pretty brutal, huh? Well, sheltering down periodically can mitigate this damage, but how long will food last in this harsh winter? What if we take the same situation and throw in a PC Ranger or Druid to help? Instead of 66 damage after 8 days, the party only takes 18. So there is plenty of room for a party to conquer the elements as they reach high level and stay prepared.
Overall, the numbers fit well enough for my liking at the moment. Reasonable travel conditions permit safety up to a 75 mile round trip and judicious use of stops along the way in an inn or even a camp make long distance traveling feasible. Yet, the truly daunting terrains pack quite a punch even for high level parties. Just for fun, what happens when traveling across the Sahara in summer? Well, by day 3 they are doing ok and have only taken 2 damage. After 5 days of baking in the sun, they just took 15 damage (23 total) and a fool who wants to spend a week takes a hit of 94 damage. Realistic? Well, deadly for sure...
Feel free to let me know what you think in this post or the previous one.