Both me and Mr1470 regularly run LDWA challenge events. Having a note of the time you've taken is an interesting marker to your fitness levels but imho little more

Challenge event organisers and walkers don't have a problem with runners although there are a few events that exclude runners

If you want to race then enter a race. If you want a well organised challenge event where you often team up with other runners / walkers then do one of the LDWA events. Personally, I don't think 'racing' fits into the ethos of these events (well not until there's someone overtaking you who finished later than you last time out)