Almost all races have posted time limits on their website. You need to estimate your finish time and find a race that has an appropriate time limit. If you can't find that information, email the race director. It doesn't matter if you walk or run, what matters is that you cover the distance in the allowed time. Good Luck and congrats on making the commitment!
"I can do all things through Christ who strengthens me." Philippians 4:13
"Pain is temporary. It may last a minute, or an hour, or a day, or a year, but eventually it will subside and something else will take its place. If i quit, however, it lasts forever..That surrender, even the smallest act of giving up, stays with me" Lance Armstrong
Have you tried a Google search for HMs in the UK? A search should return sites with race calendars (like Active.com in the USA and I think it has a global version too) that you can sort through and link through to the individual race sites to see their rules.
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