Looking on the internet deeply has found these results:

I used to keep fit is the most popular phrase on the web.

I didn't use to keep fit 10 results on the web	I used to keep fit 1177 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	I used to keep fit, mainly by jogging and cycling, and occasionally I think about resuming these activities! Heather Turnbull, Citizen Advocacy Development�... Aug 2, 2013 ... I used to keep: fit by running and going to loud step aerobics classes. Now I gain strength (mind and body) through yoga. I used to believe: that I�... I used to keep fit by biking to work, and it took a while to find a new routine that I liked as much as that. But like Meg, it keeps me sane and it's been worth making �... Before my children were born (Katelyn 9 and Mitch 4) I used to keep fit by road cycling in the hills and hiking the moors around Lancashire and Yorkshire before �...

I didn't use to keep fit

10 results on the web

I used to keep fit

1177 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

I used to keep fit, mainly by jogging and cycling, and occasionally I think about resuming these activities! Heather Turnbull, Citizen Advocacy Development�...
Aug 2, 2013 ... I used to keep: fit by running and going to loud step aerobics classes. Now I gain strength (mind and body) through yoga. I used to believe: that I�...
I used to keep fit by biking to work, and it took a while to find a new routine that I liked as much as that. But like Meg, it keeps me sane and it's been worth making �...
Before my children were born (Katelyn 9 and Mitch 4) I used to keep fit by road cycling in the hills and hiking the moors around Lancashire and Yorkshire before �...