How long does it take you?

Assuming it's in the 30-60min duration range, I'd say to increase the number of days you do the course, or do it twice a day (e.g. morning and evening) instead of once, rather than increase the distance.

Walking on the grass rather than the path may be an alternate change (me likes a lot unless wet). The softer ground should help reduce the strain on your joints and backbone while simultaneously increase the strength needed in your feet and lower legs, and exercise different parts of your walking/running apparatus (word?).

Id say stick to current course till you can do it comfortably at constant pace without any stoppage. Seems you are not far off from this on a good day. Once ok use that as a baseline to build, either add km to for distance or increase pace same distance, depends what you want to achieve... you can do 1 day longer distance and another day same distance faster pace