I also like the journey being an adventure itself, but I don’t think it’s well supported by any RPGs that I’ve seen. The thing about journeys, especially in premodern times, is that they can be challenging, dangerous, and can kill you even without monsters. Easy examples are the journey through Mirkwood, the Odyssey, and Oregon Trail. Heat, cold, starvation, thirst, disease, exhaustion, bad weather, exposure, falls, getting lost, insects, and spoilage can all be lethal. While I don’t think dying of dysentery 50% of the time a la Oregon Trail would be fun, I would at least like to see a nod in this direction. I think that making this happen requires mechanics built around food and water supplies, money, foraging, weather, and fatigue, and adventures that have meaningful decisions based around these factors. Add in the monster, traps, and meeting interesting and exotic people that are already found in most RPGs, and you can have a journey as an adventure.