In reply to elliot.baker:
>...my Garmin watch said 4.94km or something like that,
That's 60 meters. You could average your pace over a 60m stretch when you were going full speed and see if it matches the park run times.
If the difference between your PB's is a second, then the extra 60m might be a big deal. If not, then don't worry about it. Your PB is for you and you alone. If it makes you happy, then enjoy. It's not like Park Run is the entry qualifier for the Olympics. One year the Manchester marathon was 380m short. That was a race qualifier for some people, and for others, who may only do one marathon in their life, it wasn't a proper marathon.
If you are really, really bothered you could walk the route. There will be more data points, so a more accurate distance can be measured. Or you could run faster next week