I have trained and ran 5 marathons on T-bol. I used a low dose of 20mg a day though. I am 181 pounds so I didn't need much. I also weight train and cycle long distances. I used it mostly for recovery but was happy with the extra endurance it provided. I have been able to keep most of my strength gains and endurance.
Endurance has more to do with VO2 max, which has very little to do with the steroids taken. Your oxygen uptake will probably remain the same during PCT as long as you keep up your training.
You may loose some of your muscle during PCT, but weight is not a good thing for a distance runner. Your endurance may even be better after pct than during your cycle.
^^^ more concerned about lower testosterone levels during the PCT and until the HPTA starts working again afterwards. don't see how it wouldn't slow you down having low T... you are going from an anabolic and androgenic state, to a lower then normal state. then going back to equilibrium so a loss of strength and speed has to be necessary
would love for someone to run AAS who is a 5K or half marathon athlete and document their experience with links to their race times as proof that AAS cycles made them faster. as long as they were already a competitive runner and not starting out. I will give them 5M karma if they put up a full log.
I think the point you made about them already being a well trained runner is the difference. A lot of people will supplement aas to get them to a certain point perhaps but once you are at the top of your game I cant see it helping much.
I do know cyclist and triathletes who cycle on and off throughout their season and training. I do think its more for recovery and just to feel good. I know they are not big dosages either. Ive heard test base with some deca and eq. These guys were not looking to get bigger just stronger and faster. Not sure if it was the best route for them but they did it.