It is great that parkruns are getting more diverse in participation. I did my first one in 2015 in 30:30 on a hilly course... and was in the last 5 finishers! These days that's a mid-pack finishing time.
There's a 95yo that parkruns/ walks near us taking about 1:05:00 which is amazing. It's a flat course, popular run and not an outlying time.
I do think it's worth considering volunteer time if you know you're going to be a significant outlier on a parkrun's usual times. It takes a lot of people giving their time in the morning to make parkrun happen week in week out and filling the roster is one of the most stressful parts of the Director's job. My nearest run has parking permission until 10:30 and it's a 5 minute walk round to the parking, plus putting equipment away. Someone taking longer than 1:15:00 (which is a very slow walk 25min/ mi ) then starts putting parkrun into an awkward position with their arrangements with the council that could hypothetically result in those arrangements being revoked. Volunteers are there from 8:30, that's coming close to 2 hours standing around. In winter/ difficult weather, that can be a health and safety issue (that can include summer heat too). People have other arrangements in their day that they'll make around the usual times that their run takes. Some volunteers have tasks such as results to complete afterwards. There's also the practical concern at the finish line that an incident may have occured on the course if marshals/ participants aren't returning when you'd reasonably expect them to. While there is no official cut-off time, there can be deadlines on arrangements that parkrun has based on reasonably anticipated finish times.
I've walked parkrun many times through injury or when dealing with the whims of my children when they were younger. I'd always say to look at the times; it's more comfortable to not be a significant outlier anyway. Courses with fewer laps are easier for walkers. Having laps 2 and 3 storming behind/past you can be a little stressy, single lap or out and back is easier. Generally people get fitter by participating and their times will improve. At 45 mins there'll be a cross-over of brisk walkers (4mph) and slow runners either a constant slow pace or run/ walking.
Walking at parkrun is great, and most people can walk at a moderate pace that is reasonably forseeable, but there does come a point where if someone is significantly slower than reasonably anticipated, it does have consequences on the volunteers and parkrun itself, and it's a valid logistical point to raise. While parkrun times have increased (brilliant!), it's been a gradual process over years.
Not saying that OP is in extreme outlier territory, have a great time, it's a lovely community event.