Plain-looking office lady feels insecure about her own body and signs up for some fitness but her training changes her more than she imagined.

It's one of the older works of Marsa, original japanese version was uploaded previously here:
/g/1435466/7bfa6efa62/

This is the english translation that Marsa released around half a year ago.

You can read for free Marsa's works on pixiv here:
https://www.pixiv.net/users/375450
If you liked his works please support him on Fanbox here:
https://marsa.fanbox.cc/
It's really cheap, just 2-3$ per month and you get access to all that he released there.
He also has account on twitter:
twitter.com/marsax_

Gotta say a good story but the repeated blackout and blurred pages become annoying very quickly

Base +6, akb568 +6, dposer +6