Am I reading you right? Your long run is 20 miles and you are hoping to get it to 30?
I would stop at 20, possibly 22 max and even then only perhaps once a month. Slow is good though. I would try and squeeze a medium long run (say 10 miles) in once a week if you possibly can. Some sort of speed work is good, I do Parkrun too but as part of my 4 runs a week schedule. I don't have a marathon on the horizon so I am only doing 20 miles a week in total. That will start to build next year.
Park run this morning on Wimbledon Common. Soooo muddy, it's like it is still February. No wonder the tennis is behind schedule. No PB today, way off the pace. Even without the mud I don't think it was going to happen today. Still recovering from the holiday!