Wow, I can't believe I have something useful to add to this forum given my current horrific shape and fitness levels!
Still... After I got shot in the USMC, I was on light duty for an extended period of time. After I got to the point where I could run and whatnot, the battalion commander decided that it was time for me to do a PFT just to see where I was. The run part was horrid, to say the least. My company XO took an interest in getting me back up to speed, and so I went running with him on a daily basis. The one thing that he did that signifigantly increased my endurance and run times quickly and efficiently was to do fairly long (about 100 yds, give or take) sprints in sets of 10 after each run. I won't make any bones about it, it was awful and physically challenging, but my run times improved very, very rapidly. So much so, I ran the fastest three miles (19 minutes) of my entire USMC career only a month after failing to break 30 minutes for three miles. You can also incorporate a technique known as "fartleks" to increase your speed and endurance. Simply put, when you run, use powerpoles as units of measure. Run your normal pace between the first set of poles. Then, for the next set, increase your speed. Then the next set, ratchet it down. Then sprint, then slow down, then whatever. There is no particular "right way" to do fartleks other than to vary your intensity between sets of poles (or whatever measure you use). It is a good way to work to build strength and build in variety over a normal run. I don't think I could run more than about 100 yards now without having a heart attack, but back in the day, fartleks really, really helped me in my running.
Also, to answer your other question. I was an 0331 (machinegunner) and I chose infantry when I went into the USMC. Fact was, at that time I was interested in the physical nature of being a infantryman, I was interested in the tools and tactics of the infantry, and I like guns. Really, thats what it boiled down too. To outsiders, it looks like a horrible life, but when your in it, it can be a very challenging and fun job to have. Yes, it can be very hard. Yes, it can be dangerous. But you also get to do a lot of neat training and I believe that the camaraderie that an infantry unit possess is better than any other. It's like this man: I look at guys that fight forest fires and think "Boy, that job would certainly suck", but I know guys that fight forest fires and they flat out love it. Even when they have been up for three days humping crap up and down a hill, they just love it and wouldn't want to do anything else.
What it boils down to is mindset. You are capable of pushing yourself far beyond what you think you can, both mentally and physically. You will want to quit long before you physically have to quit, and you have to learn to mentally power through those times when you want to quit.