Cool question, and potentially a pretty cool world. :-) A few responses to your proposed planet and civilization, working towards questions of transportation:
Planetology of a forest world
There's a bit of a problem in your initial description of the planet:
Imagine a dense deciduous forest, with trees packed close together, stretching across the entire planet. The planet itself is Earth-like, and has Earth-like vegetation, trees, and forest creatures.
There's an inherent conflict here: An Earth-like planet, with "Earth-like vegetation, trees, and forest creatures", is not at all compatible with the concept of a more or less uniform, world-spanning forest of deciduous type. There's simply too much variation in climate, terrain elevation, weather patterns, and distribution of moisture to permit this kind of thing, ecologically speaking.
You could address this by postulating an incredibly dominant tree species (or stable group of collaborating species) for your forest - capable of establishing terminal forest conditions in everything from arid cold weather to hot humid areas, and managing elevation without difficulty. Alternatively, you could modify your world to reduce the tendency to climatic variation. I'm guessing that the former would be less difficult. You may want to do both.
(Of course, you could reduce the area of coverage of your forest, leaving parts of the world empty of the human habitation that depends on the forest for housing, food, and materials...)
Surface travel
So here's a world-spanning forest. Forested terrain isn't easy to traverse on foot unless there are known pathways through the trees. Pathways provide not only easy passage through undergrowth, but also are navigational aids: if you are traveling under a forest canopy, it's terribly easy to get lost unless you are following a trail. I think your serious consideration of tree-level bridges is a good acknowledgement of this... 258
as a corollary, your stone-age/early iron age civilization isn't likely to have the resources to keep very many surface roads clear of forest regrowth. One perhaps significant question: are their areas of forest, similar to terminal pine forest in our world, that suppress undergrowth and leave a pleasant and easily traveled forest floor? I realize that you've specified deciduous trees, but I don't think the ecological dominance of pines is limited to conifers... These areas would make a large difference in traversability on foot.
Water transportation
One of the most important remaining topographical questions is, What are the bodies of water like?
For a lot of purposes, water transportation is the cheapest way to move people and goods over distances. Navigable creeks and streams can handle canoes or similar; larger rivers permit rafts and flat-bottomed boats; lakes (even narrow ones) will support open boats, rowboats, sailboats; larger lakes and seas give you larger vessels and kayaks.
Most of these vessels are attainable by stone-based technologies, and easier still if you have iron tools. Consider the birchbark canoe, the dugout, the curragh and kayak made of waterproofed skins stretched over frames of bent branches.
Large planks, for larger vessels, are not typically something we'd associate with stone technology: however, the easiest to build are riven (or "reeved") planks, which are excellent for clinker-built (aka "lapstrake" hulls; reeved planks are are apparently within reach of a determined stone-age woodworker.
Air transportation
When it comes to your interest in gliders, I tend to agree with <PERSON>'s concerns about the difficulties of the initial leap in concept. I do not think the means of building gliders would be outside the reach of a patient and craftsmanlike stone + iron civilization
There are a couple of planetological tweaks that could make air travel more plausible:
* Denser atmosphere makes for easier winged flight. <PERSON>, in The Man Who Counts, made the atmosphere of the planet Diomedes quite thick in order to support the weight of the intelligent winged race his human characters were dealing with. (And because he was <PERSON>, he also took care to describe the unfamiliar behavior of things like sound and wind in that dense atmosphere.)
* Large flying animals would make ready mental models for flight on a human scale - the kind of thing that gliders would emulate. Could your world (especially with a heavier atmosphere) host a lot of large soaring birds/bats/pterosaurs/whatever? (Of course, if it supports enough of them, the temptation for people to ride them would be significant... ;-) 87