Here are some of my thoughts on speedwork. Most of what I learned is based on training with Rick, but I've started to take some liberties with his wisdom to create my own speedwork philosophy. This is based on my experience of training to run marathons and ultra marathons as a novice runner. If you've been running a while you've probably got your own rules. If you only want to focus on speed and not building distance your results may very with this advice.
Speedwork should be no more then 10% of your weekly mileage. If you want to run 3 fast miles during the week you'd better be running a total of 30 miles for the week. Most beginning marathoners don't run that much when they start a training program so I would recommend introducing some faster miles around halfway or two thirds of the way through a training program if things are going well. Breaking the 10% rule tends leads to injury.
Exception: After my first marathon I kept my speed work miles constant while decreasing my total weekly mileage. My body was used to the 1 fast workout a week already and I was focusing on running a fast 5k.
Tempo runs: For me a typical tempo run is 1 mile warm up, 1 mile cool down and the miles in the middle are run starting near a 10k pace and getting progressively faster until reaching around a 5k pace. The warm up can be run at any pace you want. The mile cool down is slower then your fast miles, but still at a good pace, generally a bit faster then your long run pace. You want to teach yourself to recover at a faster pace then you normally run at. This is probably the easiest workout to do, all you need to do is track how fast your last mile was and then run faster then that. Only the middle miles at the faster pace count towards your 10%.
Tempo Intervals: Typically I'll do the same 1 mile warm up followed by half mile intervals faster then a tempo run separated by a half mile of recovery run near my long run pace. I don't do this workout as often as a regular tempo run. I count both the half mile at speed and the half mile recovery towards the 10%, the warm up does not count.
Hills: Again fairly simple to do, unless you live in the middle of the flattest part of the world like I do. Fortunately overpasses work just as well. I'll do the standard 1 mile warm up then start running across the overpass. The goal is to work hard going up, and keeping the same speed(decreased effort) going down. You teach yourself to recover at faster paces and running down hills is good practice as it works different muscles then running flats of uphills. I try to do this just a bit slower then a tempo run. If you have a big hill just turn around rather then run all the way to the other side. The overpass I use is about a third of a mile from end to end. I count any miles on the hill or overpass towards the 10%, I've got a flat mile loop I use for the warm up that does not count.
Benefits: There's more to speedwork then just having fun torturing yourself by running uncomfortably fast. You lower your heart rate, improve running efficiency, improve your ability to take in oxygen and you get faster. Not many people enjoy speedwork, but the payoff is well worth the effort you put in.