Crumbs that's long. My 2 are at independent preps, and although they run "all day" sports days actually they don't start till 11, after the littler ones have had their sports. Plenty long enough for me ... (one bout of sunburn and one soaking resulted this year). And that's with kids who are merely non-sporty rather than having additional difficulties.
Have had the letter about sports day today. It is all day, KS2 track events morning and field events afternoon, this is just a mediocre state primary. Parents invited to whole day and they will be locked into school grounds from 9.30 to 3.00. Lunch is picnic on the field for children to be with their families, but lunchtime supervisors will be there for children whose family cannot attend. No playtimes or breaks.