In about 7 months I wanted to take part in an activity that requires me to hold 25 lbs of metal awkwardly out in front of my chest for long amounts of time (while being physically active).

I do a little bit of running and am moderately in shape (I have a fast metabolism), but I have almost no upper body strength. I’ve recently started to do some rowing on a Bowflex machine.
Are there any suggestions on things I can do to build endurance and upper body strength?