Signed up for my first parkrun today - Never run more than 2.5km

I'm a 28M who doesn't look in bad shape but my fitness has always been awful. Recently I've been trying to get into running as injuries have put me out of other sports.

I've been doing short 1.5km-2.5km slow jobs and finding those tough with the aim of doing some 5km parkruns. The plan was obviously to build up slow but I kind of had a fuck it moment and just signed up for this weekend.

I know I can always walk bits if needed and I'm not looking for a great time. Any advice for a first parkrun and making the leap from 2.5km to 5km?