Excellent thread. Luckily, I usually do not have to run at FRA because it is my final destination and I can start in town. But I did some runs on long layovers in the past. So just one thing to add to your first post: I would not bother to take the bus to get the the hotel and start from there. You can directly step out of the terminal and start running. There are excellent pavements that will get you to the forest north of the airport in no time. If you start from T1, it is best to turn left, run past the FCT, LH aviation center and LH flight training and turn right onto the bridge (Moerfelder Str.) and you will find a nice trail on the right hand side once you come of the bridge. Its about 2k (a bit over a mile) from T1.

From T" just step outside, take Kapitaen Lehmann Strasse until you get to a large roundabout, take the exit northwest (Unterschweinstiege) and voila! It is about 1k or 0.6 miles from the Terminal to the trail.

Running directly form the terminal will save you some time depending on your fitness and your starting point you will hit the trails in 4 to 10 minutes.