Your dog is fit, leggy and apparently tireless, and a proper day out looks tempting. The problem is that they will not be the one to call it.
An adolescent dog is still growing, still filling out, and still finishing off joints and growth plates. They are also enthusiastic enough to hide the cost until later.
Build it up rather than jumping to it
Take the longest walk your dog already does comfortably as your starting point, not the distance you would like to do.
Add a small amount at a time, and hold at each new distance for a week or two before adding again.
Change one thing per step. Longer, or hillier, or rougher underfoot, not all three on the same day.
Plan an out and back you can cut short, rather than a loop that commits you to the full distance.
Build in real stops. Sitting for 10 minutes with a chew is part of the walk, not a failure of it.
Terrain counts more than distance
Two miles of steep, rocky descent asks far more of a growing dog than four miles of flat path. Downhill, scrambling, deep sand, loose scree and repeated jumping on and off things are the demanding parts.
Pick paths over rock and scree while your dog is still filling out.
Lift or route around big drops instead of letting your dog jump down them repeatedly.
Watch the pads on gritty or hot ground, and check them at every stop.
Carry water and offer it often, and turn back early in heat rather than pressing on.
The next morning is your measurement
How your dog looks during the walk tells you almost nothing, because adrenaline covers it. How they get off their bed the following morning tells you a great deal.
Look at the first 20 steps of the next day. Stiff, slow, reluctant to move, or lying down more than usual means yesterday was too much.
If the day after was rough, drop back to the last distance that was fine and stay there longer.
Two easy days after a big one, rather than a repeat performance.
Keep a note of distance and how the next morning went. A week of notes is more useful than any number someone gives you.
Safety note
Pain frequently shows up as a change in behaviour rather than as obvious limping. A dog who is suddenly grumpy, reluctant on stairs, slow to get up or unwilling to walk should be seen by your veterinarian. Speak to your veterinarian before you start building distance, especially for a large or fast-growing dog. The same goes for any dog with a known joint or heart condition. They can tell you what is reasonable for your dog's age, breed and build, which no article can.
One thing that surprises people
A long hike does not reliably produce a calm evening. Fast, repetitive exercise can leave an adolescent dog wired rather than settled. Sniffing, chewing and food puzzles do settling work that mileage does not.
Today's version, in one line: repeat your dog's current longest walk this week, and look at how they stand up tomorrow.