Good preventive care before the event. If you’re not already doing it regularly, have your vet out to do a thorough physical exam and flexions (three weeks out is an ideal time). Head off any problems before they become lamenesses and identify any areas that might need special attention. Same goes for hooves- make sure your horse is on a good schedule and set him up to get new shoes a week out from the first jog, so they are tight but it is long enough out to identify a hot nail, etc. Make sure your fitness work leading up to the event is appropriate for the terrain you will be running on (deep sandy footing, hills, etc) so your horse is well prepared and accustomed to the demands.
Other than icing and walking/grazing, I like to pack hooves (unless your horse has pads) and I keep the horse wrapped when not icing. Poultice is good, as are therapeutic wraps like magnetic or Back on Track (bonus of no washing poultice for icings). Last but not least- let your horse have some rest! Walking is important to prevent stiffness, but they also need to rest up and be fresh to jump the next day. Good luck and have fun 