Try running backwards!
Most of you who do functional fitness with me run plenty. Recently I did a workout where I alternated running forward with running backwards. You should try it.
Running forwards works the back of your legs and your hamstrings and calves. Running backwards really works the front of your legs, meaning your quadriceps and still works the calves along with it.
If you are lucky enough to have access to a hill, there is an even better workout waiting for you. Run down the hill forwards, and up the hill backwards.
This will really give you the extra strenuous workout you are looking for. Here are some more of my recent favorite workouts.
While wearing either a weighted vest or a chain vest, do a ladder of pullups and giant stepovers. I call a giant stepover the move consisting of stepping over a plyometric box at least 18 inches tall. See how many steps up on the "ladder" of reps per set you can do. A good start is to do 2, 4, 6, 8, and 10 reps.
A longtime favorite workout of mine is to run down and up the hill once, and do one rope climb. If you are younger than me, you may want to do a pair or more of the rope climbs for each round. See either how many rounds you can do in a set time or how quickly you can do a set amount of rounds.
Whatever you do in your running workouts, mix them up. And, on any given run, mix it up. Do not just run constantly, even if running is all you are doing for that workout. Run harder for a while and walk some. Allow your body to do a mini-recovery in the middle of the workout. Then when you resume the running, you will go harder.
A sandbag workout I keep going back to consists of only two exercises, both done with a small sandbag weighing about 40 pounds. This workout is 3 to 5 rounds of 10 reps each. The exercises are turkish getups, done half with each leg, and pushups, done with the small sandbag down your spine. This workout works your whole body.
If you have access to some flat pavement, which I do not have, drive to it and push and pull your vehicle. To pull the vehicle, use a 3/4" diameter rope and a 18 wheeler truck inner tube. The inner tube goes around your waist and you attach the rope to it and the bumper hitch of your vehicle. Do not attach the rope to the bumper. You will pull it off.
I hope these give you some ideas as to how to do your functional fitness workouts. My favorite place to do workouts is at home. There is no reason why you cannot also do your workouts at home. Keep it safe. Keep it fun. See our sandbag training page here.