I'd use tail off.
tail off — phrasal verb with tail verb: to reduce in amount or become
lower in level
M-W adds the 'gradually', probably connected with the tapering of a tail, though doesn't give a wide enough definition:
tail off phrasal verb: to become smaller or quieter in a gradual way
ditto for Farlex:
tail off: to dwindle, diminish, or fade away.
His campaign started really strong, but public support for the candidate tailed off following a series of scandals.
But they do include an 'agent [here non-sentient] reduces [here perceived] output' example:
The lights on the car began tailing off into the blackness of the night, leaving me alone in the empty field.
'Tail off' is often used to describe speech, even as a quote verb. But none of the dictionaries I've checked in mention the running sense, but it is quite common, especially in horse-racing. Here is one example [LivesRunning], involving a human agent:
He attacked the field, running the first lap in 58 seconds (as fast,
by comparison, as the women’s Olympic final), and, if he tailed off
afterwards, still he just about held on to finish in around 2 minutes
It's not much of a further metaphorical stretch to apply it to gradually falling away from other exercise (and then any other practice). I've only been able to find an anecdotal example from mumsnet on the internet:
I've been to the physio, and started off doing my exercises
religiously. Have tailed off somewhat now.