DFW is an odd case. Terminals A,B,C,D are connected by walkways, but terminal E isn't. And the walkways are all airside; there's no way to walk between terminals outside security.

ATL may be walkable, but it isn't walk-friendly. It is probably close to 2 miles from the main terminal security checkpoint to walk to the furthest gate.

FRA is incredibly confusing to the unfamiliar, despite looking "friendly" on a map.

Walk-Friendly Airport list?
 

SJC and BOI

Are we using "terminals" and "concourses" interchangeably? If so I believe these are walkable airside to all gates:

ATL (as noted, though, can be a looooong hike).
SLC (I think)
HNL

I just did a walk from OKA domestic to international. It's probably 400m on flat ground (which allow you to bring carts to and fro) and they still run a bus...

I'm not sure I understand this thread. How can any airport "force" you to take a bus or train? You can always walk out the door and walk to the other terminal. It may be far, and there may not be a road, but I don't see why you couldn't do this at any airport.

I have been to dozens of airports, and I can't think of any where you couldn't walk between terminals if you had the energy to do so. There's no reason to design an airport like that.

Between CDG1 and CDG2 there is no walkable path I'm aware of. As well as to CDG 2G. Also some L and M gates at CDG 2E are only accessible by an airside train shuttle.

In some places you would have to walk on a highway that is dangerois and forbidden to pedestrians :(

You wouldn't want to try to get to the Heathrow CTA (Central Terminal Area - T1-3) on foot either. There's a single set of tunnels in and out. You did used to be able to walk down the service tunnel (originally reserved for taxis and cycles, later opened up to private cars) but that's closed off to pedestrians and bikes now.

By contrast, Schiphol has a rather excellent cycle/pedestrian tunnel under the taxiway, to take you down towards Schiphol-Noord and Badhoevedorp.

On a long layover that can be a plus though. I've killed many an hour walking between concourses at ATL. The only thing I dislike about them is that they can get quite hot and muggy during the summer.

PHL, sort of. Its one of the longest airports in either the US or the world according to some mag article I read on a recent flight.

It is a nice long walk between A and C. F requires a bus ride.

SJU although its more of a hub spoke design but the end of D to A is a workout.

That should improve soon with the linkage of T1 and T2. What will be left is intra-T1 and T1-Int'l connections.

LIS does not have a T1>T2 connection, a bus is required. (Theoretically you can walk along the road, but there are no sidewalks). In fact, T2 is not accessible by foot at all.

MAD T123 are very well connected, but T4 is away from everything. Probably accessible by foot, but I haven't tried.
BCN's two terminals are a few kms apart but theoretically it's possible to walk from one to the other.
LHR obviously is theoretically walkable, practically miles apart (T2 and T3 are well connected).

F only requires a bus if you wish to stay airside. It is walkable landside.

CLT is walkable. There is no train at all.

And to the OP's question about stairs...I do take the stairs (down atleast) if I check my bag. It is a pain to do stairs with a rollaboard.

Walking through the garage at LAX seems to require at least some short flights of stairs. It can also be confusing. Unless you need to do something like getting between T1 and T8, I prefer to take the path following the sidewalk in front ot TBIT.

A lot of smaller airports are one building or connected buildings, even if different sections aren't connected airside. SYR, MDW, PIT, BDL would fall into this category.

STL too, except for what seems to be a separate terminal for Southwest, which would put STL into the same group as MSP and DTW.