I would speak with your confessor first. Then your doctor. Just to be safe on both ends. :)

I am often forced to fast. I offer it up (as usually I am in a lot of pain those days too) accordingly. If greater merit for those souls in greater need is granted because the day is a Friday (or any of the rogation days) God be thanked.

If you were to choose another day  other than Friday, my choice would be Wednesday, as this follows the practice of Traditional religious communities.

I will take any physical/medical benefits that come with fasting, as a secondary to the spiritual benefits.

I don't see any problem with it.  I think of it more like you are sanctifying a water fast by doing it in union with the Church's Friday abstinence & Our Lord's Passion.  

This.

Anything longer and the concern is damage to your tendons or ligaments.

.
Not true.  
.
There is no danger to your body during a fast, unless you
have diabetes, cancer of the kidneys or possibly some other
serious condition.  
.
Fasting is not starving as many people think.
.
Medical doctors know nothing about fasting beyond 24 hours.
They used to teach in medical school that fasting for 10 days
would kill you.  
.
Dr Shelton was the expert on fasting when he was living.
He had a fasting place in San Antonio, TX for many years,
where people would fast 14 to 60 days.  I fasted there 3
times.  I met a guy who fasted 63 days.  The longest fast
at that place was 90 days.  
.
I have fasted 20 days, 25 days and 30 days at different
times in my life.  My longest fasts were done at a fasting
retreat place.  The only thing I had while fasting was distilled
water.  
.
Dr Alan Goldhamer has a fasting place that I recommend:
https://www.healthpromoting.com/
.
When fasting at home, you can fast 3 to 7 days, but you
need to learn how to break a fast.  You start with watermelon
4-6 ounces every two hours (fruit is OK also).  It takes an
equal amount of time to break the fast.  Fast 7 days, then
breaking it takes 7 days.  
.
Fasting one day a week is OK, unless you have a serious
condition like diabetes.  Even with diabetes you can fast at
a fasting retreat place under supervision.
.
Fasting Can Save Your Life is a good book to start with:
https://www.amazon.com/gp/product/0914532421/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=0914532421&linkCode=as2&tag=httpwwwchanco-20 />.
The first 5 days of a fast, you lose the most weight, usually
2 pounds per day.  After that ... 1 pound per day.  For this
reason some people like a 5-day fast.
.
The longest fast on record by a human is 132 days.
Snakes have been know to go for 2 years without eating.
.
The hardest part can be breaking the fast.  You have a
tendency to eat too much.  One man fasted 24 days and
then went to a restaurant and had a large steak dinner.
He died in the hospital within 2 days.
.

https://www.drfuhrman.com/retreats/17/california-summer-health-getaway


https://lifestartretreats.com/locations/weimar-ca/

Catholics shouldn’t do yoga because it could open doors to demons.
Maybe healthy eating with concentration to avoid sin should be more encouraged during sermons.   
St Ignatius silent retreats is a good way to pray and fast.  Too many Catholic functions and events focus too much on really unhealthy food.  One chapel there are donuts, coffee, cookies and hotdogs chips after Mass every Sunday which is not healthy.   Pancake breakfasts are really unhealthy and there are many who eat without grace by priest at one traditional chapel. And portions are out of control.  It is gluttony. 

SSPX St Ignatius silent retreat we were taught to have reasonable portions.  Mortification.  To stop eating when we had enough instead of being stuffed and over full.  

I forgot to mention about listening to recording about young girl in Hell as we ate really helps too.  

That recording is called, "The Story of Annette, a Soul in Hell"
and you can read it here:
.
http://drbo.org/annette.htm

That recording is called, "The Story of Annette, a Soul in Hell"
and you can read it here:
.
http://drbo.org/annette.htm

Wow. Thank you for posting that.
Tempis fugit, momento mori

I don't see any problem with it.  I think of it more like you are sanctifying a water fast by doing it in union with the Church's Friday abstinence & Our Lord's Passion.  

Thanks for this.  It is very helpful and inspiring.