I know on first impression this will sound unhelpful, but just run faster mate. You CAN do it, you just don't want to. No matter how bad you feel, you CAN run faster, you're not *text deleted* die! And if you do, you won't know about it anyway so either way you win!!
Interval are more structured generally, but there are no hard and fast rules.
An example would be run 400 m hard and then walk or jog for a minute to recover and then repeat as many times as you require.
Fartlek is simply running at faster pace during a run (for however far you you decide) and then slowing down to recover then repeating.
Basically, both methods train the anaerobic system through using repeated bouts of high intensity running (or cycling, swimming, rowing etc).
Your best bet is to do a quick google for more info.
Ref - How many times per week ...
It depends on a number of factors, such as -
Current level of fitness.
The intensity, duration of the interval / fartlek sessions.
The event you are training for.
The demand other training sessions place on your body.
Time away from event (prmc) you are training for.
Plus other factors.
As a general rule, replace one of your normal runs each week with a interval / fartlek session. Do this for 3-4 weeks and then replace another of your normal runs with a fartlek / interval session.
2 anaerobic sessions should be fine for MOST peoples needs.
There's loads of info on the web so you should be able to get some more detailed guide lines / advice.