Cannot find the meaning of this expression anywhere.
No joke answers please. I am sure they will not be funny.
It means becoming overweight in the context of age plus inactivity. Becoming overweight, but usually in a person who has not been overweight for a long time.
It’s also not an Americanism (to head you off at the pass there), I hear it in Sydney quite often, and have found cites of its use in the UK.
“John used to play football and be very fit, but now he is almost 40 and running to fat.”
Is that unfunny enough for you?
Cheers,
G
I thought that it was a British expression but a bit old-fashioned
And I thought it was an American one, because I heard it there quite often. A quick Google search for “running to fat” and “run to fat” show equal entries, give or take, for both British and American sources.
Cheers,
G
I have heard it used occasionally.
I take it to mean someone who used to be fit (or at least exercise regularly), but who is now getting fat.
How about ‘running to seed’?
I use this to mean someone who is becoming dishevelled or looking much older.
Yes, I hear running to seed colloquially to mean just that, Glee.
You have the same meaning for “run(ning) to fat” as I do, from an American perspective, and the same meaning as I hear the phrase used here.
Cheers,
G
You don’t “run to seed,” you “go to seed.” [/nitpick]
Um…yes, in the US I heard “go to seed” but here you “run to seed”.
Cheers,
G
Gah – y’all are so impatient!
Isn’t there also the implication that the person has replaced some muscle with fat, while staying near the same wight, rather than merely gaining a lot of fat?
I think it can be used in either sense.
It is an expression, rather than a definition…
Excellent.
And I like your username too. 
Over here Philadelphia is a cheese. 