I too am a runner (~100 mi/week when not tapering or recovering) and "normally" I limit my caffeine to a double espresso at 5:15am. I take that one to synchronize my sleeping and bowels. However, it's a little more complicated than that because I divide the year into three different major training blocks and what I do varies both by which major block I'm in as well as what I'm doing within that block.

As such, there are trainings where I use extra caffeine for a variety of different purposes, e.g. to get maximum speed to train my body to go faster, or for the mild analgesic effect to counter some discomfort from long distances. Depending on how much extra I take and when, I may have to take the attendant tiredness into account when planning the following day's activities, both physical (more training) and mental (programming).

I believe I benefit from my use of caffeine, but it's tricky. I log all the caffeine I take as well as all the calories, analgesics and alcohol in a text file (which I keep in a public repository in GitHub). I can then look back and try to figure out what's worked well in the past so I can do more of it as well as what has held me back.