If you own a good weight vest and want to increase the intensity level of your workout, try adding a sled harness and pull a weighted tire, sled or vehicle.
A very good weight vest is available at Weightvest.com where their weight vests are made in the USA.
For good sleds I recommend Roguefitness.com where their sleds are also made in the USA.
Start with about 20 lbs. in your weight vest and add more as your workouts become less intense. Over time you will adapt to the load and with your sled pulling efforts you will naturally gain more strength, power and endurance.
Photo courtesy of Weightvest.com, Inc.