I have really enjoyed parkrun over the years, mainly as a runner, but a few times as a walker. I'm up to about 170 since 2013. My 2 most regular parkruns have 150- 700 participants (one is a really big and busy park) and definitely have some walkers and usually a tail walker. They will also have fast runners, slow runners, people running with kids, running with prams, with dogs...
If I'm walking I can take roughly 53 minutes (brisk pace) to an hour or so. Some courses are hillier than others (and slower). All the ones I've been to are generally friendly, and I've been to about 12 around the country (and one in the USA).
Some tips:
Say hi or thank you to the marshals as you go around, stay in order in the finish funnel, remember to give back the finish token that they hand you at the end. You go up to a scanner volunteer, scan your barcode then their barcode and give theirs back, in order to get your time by email/text/on the website. Also what someone else said, please start towards the back of the group to allow the faster runners past. Unless it's a single lap course (I haven't found any of those yet, usually courses are 2-5 laps) you will get lapped/passed, so be aware and stay to one side of the path. Sometimes the super speedy folks are a bit intense.
There is normally a cafe nearby where people can gather afterwards for a chat. Volunteering is also a great way to get out and meet the organisers, runners, walkers and other volunteers.
Welcome to the parkrun community!