元々足が遅いのに年取って走るのが益々遅くなったって英語でなんて言うの？

I was never a fast runner in my youth but now I am even slower than I was
youth (noun)= the state of being young
This is a good way to explain that when you were young you were not fast and that now you are even slower than you were.

I was a slow runner when I was young, but I'm getting even slower now that I'm getting older
This is also a good way to explain it.

I wasn't the fastest in the bunch but I am getting slower now that age is catching up
in the bunch =among your peers/friends
age is catching up = you are getting older

The first phrase explains clearly that you haven't ever been a fast runner and now that you are getting older, your running has become even more slower.
To say this in a joking way, you can say "I didn't think I could get any slower" - meaning you are incredibly slow now!

"I was a slow runner when I was young, but I'm getting even slower now that I'm getting older" - In this sentence, you are explaining that your old age is getting the better of you. 'Get the better of' is a phrase meaning that something has defeated someone. Therefore, because of old age, one no longer has the strength and endurance to run as fast as when one was a youth.

Now that I am getting older I am more of a slowpoke - 'slowpoke' is a noun meaning, a person who acts or moves slowly.

*Old/Age caught up. - The opposite of old is young. Comparative Adjectives- Young younger, Old older, Fast Faster , Slow Slower,
Example Sentences:
I'm now old that i can't run as i used to do when i was young.
Age is catching up with me, I'm now a slow runner.

"I used to be a slow runner but now the older I get the slower I get as well." This explains that you were never a fast runner but the older you get the slower you run.

"Now that I'm older I'm even slower than I used to be." This is another way of explaining that you get slower as you get older.

Explanation: When you are young and healthy you are said to be in your prime.
Example sentence:
"In my prime I used to run 400 metres in 56 seconds, but age has taken its toll and of course I am much slower now."

"I was never a good runner, I'm even slower these days!" is the most casual and relaxed way of saying this.

To sound slightly more formal you could say: "I wasn't ever a very good runner, but I'm even slower these days!"

Nowadays is an adverb that describes the present time, in contrast with the past.

I thought I was slow before but now I'm even slower. - This means that you were slow in the past but now you are slower. than that.

If I go any slower I'll stop. - You would say this jokingly. If you were much slower, you would be stood still.

I was never very fast but the older I am getting, the slower I am becoming.