How to convert regular degree to distance degree?

That's a good question. The answer depends on what you're measuring. Are you talking about latitude or longitude?

A degree of latitude is pretty straightforward. It's roughly 111 kilometres anywhere on Earth. This distance stays fairly consistent.

Longitude is a different beast, mate. It's widest at the equator, also about 111 km. But that distance shrinks the closer you get to the poles. It becomes zero right at the top and bottom. So for longitude, the distance of a degree always depends on your current latitude. You can't convert it without that extra bit of info.