3 year old chocolate lab desexed then suddenly became scared of walks

I used to run her 5 to 6km and had no problems, then after the operation she wouldn't even leave house. Would just completely lie down when we tried to walk. We have a couple loud roads so she seemed to be scared of the noises.

I was able to walk her fine off leash at beach, and was driving her down daily to do that. Not really ideal to have to drive for walks though.

Took her to vet who prescribed hee some medication to give her 1 to 2 hours before walks. That's helped to the extent that we can leave house without car. Problem is on the roads she walks super slow or just randomly stops. We've gone from super brisk walks or runs to going way slower than even grandma pace. Weirdly she is happy to pick up pace when I take her off the leash.

Any ideas what's going on? Any advice how to build her back up to confidently running again. She's super social with people and dogs still so it's a shame to see her so tentative outside. I personally really like the exercise and would love to be able to do longer faster walks/runs again with her.