If it's in a portion of the resume that's clearly meant for discussing outside interests, I don't see a problem with it. (Of course, if your resume is read by any of a number of letsrun message board participants, they'll find some way to use the information to attack you, but the odds against that seem pretty high.) I've given the opposite opinion to people who want to include their Mensa membership, but that would provide much more reason to raise eyebrows.
Unless this is directly relevant for the position you are applying for, don't do it. Just write that you run as a hobby. Only bring out the subject if specifically asked during the interview.
I routinely review CVs for several positions at my firm. When someone writes that they have a sport as a hobby, I think "nice" and move on; it is a small plus. But sometimes people brags that they are competitive at a sport, by pointing out podium finishes and other accolades. In this cases, I sometimes check these out and when I find out that they are just decent amateurs, that is a minus in my book, because it shows a tendency to overstate an achievement, or (worse) to believe that an average or slightly better than average result is a great one. A 3-hour marathon is nice, but you do not want to look like a person that thinks that being 50 minutes after the leaders in a race is a notable result.
At the very bottom of my resume I have a hobbies section. It's one line and includes three hobbies of which "long distance running" is one. I see no reason why that couldn't say something like this, "...long distance running (PB 2:59:33 Boston Marathon 2022)" if that is something you want to include.
If the person interviewing you is a runner, it'll probably spark positive conservation. If they aren't, the actual time will probably go over their head and won't hurt you. I definitely wouldn't list it at the top of the resume unless your job is directly related to running or physical fitness.
A hobbies section is important to resumes. When I'm hiring someone, I not only like to know what they do a work but in their free time. Someone who takes on fitness, creative or other challenges in their free time is likely to be a very different hire, than someone who utilizes their free time in other ways.
What I've heard is that for certain finance jobs it's almost required to have an "interests" section on the resume. Most people will write something standard (i.e. running, sports, history of specific countries, etc) to not be offensive and not be dinged - it's far better for them to think you're slightly generic instead of putting something like "my interests are BLM, abortion advocacy, etc vs M'AGA, guns, etc"
Out of curiosity, what times would you deem as competitive? Is it sub 16, sub 15, or even sub 14 / sub 4?
Your boss could be a runner, and if so, likely isn't as good. They don't want all their marathon office cred overshadowed. Downplay it and just say you are into fitness.
I don’t see how boasting about something no one cares about is gonna do you any good. Just put down “Running.”
because just running is boring.
My resume would contains three interests with an interesting factoid about each. The interest section is purely for passing the “would I get a beer with this guy” test, so have some fun.
For running I don’t put I time, but note I came third in my age group at a local marathon, if any asked I’d joke the time wasn’t great, but luckily my age group wasn’t competitive (sub 25) and I can maybe win an age group again when I’m 65.
My third and last listed interest is golf, where unlike the others I just describe myself as a terrible golfer, which consistently gets laughs.