Fast running improves your running economy and makes you hurt in a different kind of way. I think hurting is the key to success in running (and often in life, really).
If you can, I recommend to do the really fast running on a track so you won't have to worry about footing.
How fast is "fast"? Try it and you will know.
I rarely go on the track alone. It's a miserable place that requires company.
How long should the reps and the breaks be? I wouldn't overthink it. If you have a group to run with, just join their workout. Whether it's a crew of milers or marathoners, most of us long distance runners neglect speed so anything will help.
Some say that speed improvements start leveling out noticeably after 8-10 weeks. This is pretty much how it always seemed to be the case for me. If you run a fall marathon like me, June/July are pretty good months for this window of speed.
Remember that it's not your goal to run an 800m PB but to hone your skills for a marathon. Every workout you do means sacrificing a different workout. There is a time for everything. Play with the variables over the years. If nothing, mere change will help you improve.
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