Much depends on your targets. A "get you round" plan will be light on "speedwork" sessions as the aim is only to build up stamina. If you're going for a fast time then yes one or two interval/fartlek/track sessions a week are pretty much essential.
For suggestions, as @Marckee says, most decent training plans will suggest something and it's a bit difficult to know what to suggest without knowing what your current pace is like, but given the distance of most of your runs at the moment then you're probably best with something short and sharp initially. Something like 4 or 6 x 400m w 1 minute recovery between each rep.
Whatever you end up doing in terms of speedwork, try and hit an even pace or gradually get slightly faster on each one; it's always tempting to go balls out on the first rep, but you won't get the benefit from the later ones if you do.