As was said before, exact speed is variable on fitness, age, terrein, what they're carrying, etc. 3 miles per hour is probably a good estimate, but you might be able to push 4. You know how in school they made you do various fitness activities like run the mile? Due to excessive lazyness, half my friends only ever bothered to walk it. One managed it in about 16 minutes (so I'm rounding up to 4 miles per hour), and another looked exceedingly riddiculous by super elongating her steps and somehow made it in almost 11 minutes. From when I walk on the treadmill before actually getting myself running, my average pace seems to be about 20 minute mile, or 3 miles per hour, but if I walked at a busy fast pace it could be faster.
I haven't personally experimented with it, but if you really want to test out different ways and speeds of walking, you could go to a gym and find a nice treadmill to play with. You can play with the speed and elevation that way. Playing with the elevation could help if you have a more mountainous terrein. Someone before mentioned that there might have been a group that ran six steps and walked six steps--why not try it out yourself, provided you have the time? With an eliptical trainer, you can also set different resistances. No, it won't be quite the same as transversing through a jungle, but if could give you some idea if you wanted to be accurate. Added bonus you can get if you go to a gym is you get to watch other people exersizing, and you can see their body types and general health (or make a fair guess).
Health and fitness are important to consider, probably more so than age. Both of my parents went on a walking tour of France, and they were some of the older people there (I think they were in their 40s then, but there was a guy in his sixties who was doing excellently), and they finished the alloted walking distance first most often. I think it was five-7 mile distances they did every day with their packs, and they finished really early in the day. Stamina is probably more important than flat strength, but then, I wasn't the one in France.
One last thing I will definetly mention is make sure your characters are wearing decent shoes. No high heels if you're having them walk through a squishy forest. The heel will sink into the ground and get stuck, even if they do walk fine. I can run in 3 and a half inch heels on flat ground, but it's still not a good idea, and my feet hurt much, much more than they would if I had been wearing nice comfortable shoes.
You probably shouldn't go into too terribly much detail on the walking, but I definetly agree that it's nice to be realistic where you can.