You all run tempos because they work. Forget about any physical explanation. It plain simple seems to work when you run a pace continuously that you race at.
You could obsess over speed and distances of your tempos.
Or you just do them when you feel like it and sometimes when others make you do them. 5 minutes at the end of a long run can be a tempo run. On the other end of the spectrum, 90 minutes are very long and you are falling off the general definition of what still qualifies as "tempo" (assuming your half marathon is 1:30h or faster).
I start out by doing 10-20 min. tempos, usually as a progression in order to be properly warmed up which is one of the key challenges for an older runner (dare I say "master runner"?). I try to build up to 40 min. at 60min. effort pace, 2-3 weeks before my key marathon. It still is one of the workouts I'm quite scared about because it gets pretty ugly after 20-25 minutes.
Update Markus: he's intrigued. I've sent him an invitation to become a writer here (again). But here's the thing: he needs a goal. I'd say this should obviously be the NYC Marathon. Handily it's the only one he would be really motivated to do right now. Trouble is that he does not have a qualifying time. Thoughts?