I am an amateur marathon runner (3:15 PR) who was looking for any legal, evidence-based edge during a 16-week training block for a fall marathon. I added 8g of L-Citrulline to my pre-run routine about 50 minutes before tempo runs and long runs.
The change was not dramatic, but it was consistent enough that I am confident it was real. On tempo runs at lactate threshold pace (approximately 7:00/mile for me), I felt like I could sustain the effort for about 5-10% longer before the wheels started coming off. The heavy-legs sensation that normally sets in around mile 8 of a tempo run was pushed back to mile 9 or so. My heart rate at a given pace was 2-3 beats lower on citrulline days compared to non-citrulline days, though this was not rigorously controlled.
The most noticeable effect was on long runs exceeding 16 miles. The wall -- that crushing fatigue that hits in the later miles -- arrived later and felt less absolute. I still hit it, but it felt more like running through thick air rather than running into a brick barrier. My average pace for the final 6 miles of a 20-miler improved by about 15 seconds per mile compared to my previous training block.
Recovery was modestly better. Legs felt less destroyed the day after long runs. I could walk down stairs without wincing the morning after a 20-miler, which was not always the case before.
On easy runs and recovery runs, I noticed essentially nothing. Citrulline seems to make its presence known only when you are pushing against your limits. If you are jogging at conversation pace, you probably will not perceive any difference.
Race day: I took 8g about 50 minutes before the gun. Ran a 3:09, a 6-minute PR. I cannot attribute that entirely to citrulline -- I was better trained overall -- but I believe it contributed, particularly to the fact that I ran the second half only 90 seconds slower than the first (my best negative split ever).
Cost-wise, this is a negligible expense for a meaningful performance supplement. I will be using it for every training block going forward.