Yes, I noticed the white outline, but the vertical line segments at each end is not that easy to see. When standing still the scale is definitely not a problem, but the readability becomes an issue when you need to take a quick look at the map while running, flying, .... That is why larger fonts, thicker lines would help.
DISTANCE RINGS:
Regarding scale: on Garmin G1000 navigation systems in airplanes, there is something similar to your "Time rings", namely "Distance rings". That is widely used to report your position relative to know points.
Could you add a "Distance rings" entry in your Map menu? The distance should then show the distance in units as specified in the Settings > Localization > Length units. To avoid interference on the map, the user should only enable one of them, either "Time rings" or "Distance rings", at a time. Alternatively the "Distance rings" could have another color and the text could be displayed at the lower half of the circle. This would be a great feature!