I've spent the last few days scouring YouTube for videos that show how to transfer at FRA using the Sky Line. Unfortunately, too many people just walk around the airport shooting video without telling you where you are or what you are seeing. Gives me appreciation for Jo's videos. She does such a good job of telling you where you are and what you are seeing.
I did find one video of what may be an example of a really challenging change of planes in Frankfurt. This one doesn't even use the Sky Line. It's a change from Concourse Z to Concourse A - all in the same building. However, it's from, I think, Z69, last gate in the new wing of the Z concourse, to A38, way out on the end of the original A concouse. It's all walking, and using the distance measurement on Google Maps, it looks like it's almost a mile's walk between their arrival and departure gates.
For those of you who might not be familiar with the layout of the A/Z building, take a look at it on Google Maps. The building today is a giant 'V' shape, but in the 2000-2010 time period, it was just a single wing jutting out onto the tarmac at 45° to the front of Terminal 1. I can remember arriving at an A gate in that time period. The door leading from the Jetway to the waiting room was closed and we were directing down a level to what was either the basement or the ground level where we went through immigration (pass port control). From there, a hallway led out to the arrivals hall and on to the Bahnhöfe.
Since then, they have added a level above the A gates for non-Schengen arrivals. They use the same Jetways, but direct you up to the Z concourse, which is essentially a copy of the A concourse, just one level up. You don't have to go through immigration to get onto the Z concourse; you are still outside Schengen. If you have a non-Schengen flight departing from the Z concourse, you can just stay on it without ever going through immigration or needing a visa.
I don't think the Z concourse was there when I arrived in Frankfurt in 2008. Since then they have built a new wing, with the same (almost) two level A/Z configuration. So, if you arrive on Z and are flying out of A for a Schengen destination or leaving the airport, you go through immigration at the root of the V to get to the A gates.
So here is the video. The couple is arriving from Bangkok, Thailand, and they deplane at the Z69 gate, which is the farthest gate from the base of the 'V' as you can get in that wing. They walk nearly a half mile (8/10 km) to the base of the V, where they go through security and immigration to get to the A level. They then walk another half mile to gate A38.
The run time of the video is 21:50, but they turn off the camera going through security and immigration, so it probably took a little longer than 22 minutes.
At 13:31 of the video, just after they pass gate A-15, they pass under a blue sign that says 'B' with an arrow to the left. The sign doesn't say anything about a tunnel, but I believe that is where you can access the tunnel to Concourse B.
The sign also indicates that gates A 16-40 and D, E are straight ahead.
At 13:53, you see another overhead blue sign the indicates gates A 16-40 are still straight ahead, but D, E is to the right, and at 13:58 you see a sign on the right, parallel to the concourse, for D, E. I believe that is showing the access, on the right, to the Sky Line station upstairs, from which you can go to Terminal 2.