Yes, it's absolutely fine to run/walk, or just walk the whole thing.
You won't be last, there's a tailwalker who goes at the back, who will either chat to you, or leave you alone in your thoughts, whichever you prefer. But you may well not be right at the back anyhow, if you think you're going to run at least some of it.
No, it's not full of very fit people. Yes, you will get some, but that's not the typical Parkrunner. It's an inclusive movement - increasingly in partnership with NHS - not just for the walking/running, but also for the community benefits (sense of belonging, increased wellbeing through volunteering etc)
The main thing is Don't Forget Your Barcode!
There will be a briefing at the start that explains what you do (start at flag; run/jog /walk - or if you were in Fulham the other week, morris dance - your way round in your own time; cross finishing line into the funnel, take finish-token as you exit funnel; get it and your barcode scanned; give finish-token back; stay for coffee maybe;look out for results email).
Have a lovely time!