Good question. Since you live in Hawaii I might have some ideas for you (unless you are already incorporating them into your programs). I usually take my clients (those you are runners or getting ready for obstacle courses) on locations where there is sand, hills and many stairs. Then I have them do a variety of exercises and routines in all of these 3 different environments because each one adds a very unique advantage to their running. All 3 are great for leg strength, conditioning and stamina. In some cases I have some of my more advance clients use a weighted ruck or vest with about 15-30lbs depending their on gender and conditioning and then take them for runs, climbing stairs and/or running hills. Very effective tool. All of these are a bit out of the box exercises and routines, but I know they work.
I also have my clients do a number of bodyweight exercises while they are running as well (maybe every 5-10 minutes depending the distance and their fitness level), because this will also improve their physical conditioning and still maintain their lean mass.
I hope this helps.