If you're stuck in a rut, feeling like you can't go any faster, there are loads of things you can do.
I find it helps to set a target. For example, if you want to run 10km in an hour, that's a 6 minute per kilometre pace (about a 9 minute mile). Once you have your target pace, you can plan your runs around that - including some running at target pace, some quicker and some slower. The variety is key - more on that later.
Ideally you need something to tell you your pace while your running. Over time, you will get used to knowing what pace you're at but, even then, it's tricky. A GPS watch is perfect because you can watch your pace on that. If you use a running app on your phone, most of those will shout out your pace to you at regular intervals. Some watches and apps also allow you to set alerts that will tell you if you go above or below a certain pace.
It's a good bet that you can already run faster than you think you can - the main barrier is psychological. Try out some or all of the things below to get over that hump.
Whatever you do, make sure you increase gradually. Don't forget the golden rule - don't increase your speed or your pace by more than 5 - 10% a week. Pushing beyond this increases your risk of injury significantly.
Variety is really important. Really, really important.
Your body is incredibly good at adapting to be more efficient. To put it another way, your body is really lazy. If you always run the same distance, at the same pace, your body will adapt. It will build the kind of muscle fibres needed to run that distance, at that pace. This way, it can do what you're asking of it with the minimum of energy used. This is fine until you come to do something else, such as running faster or further, which require different kinds of muscle fibre and require you to process oxygen in a different way.
By introducing variety into your runs, you can encourage your body to maintain different types of muscle fibre and be comfortable with supplying oxygen and energy to them at different rates. Some things you can do:
If you're training for a 10km, aim to be running 12km once a week. Running further means that you'll be comfortable with the distance on race day - you'll have plenty left in the tank (both mentally and physically) when the tiredness kicks in and it comes to that final push to the line.
Paradoxically, running slower helps you to run faster. If you do one or two runs a week at an "easy" pace, your body learns to do less with more. It begins to rely on burning fat, rather than sugar, for energy and is able to use less oxygen. The added bonus of this is that you burn more fat as well.
By "slower", I mean at a conversational pace - one where you can comfortably chat. Try slowing down to that sort of pace. Then run a bit slower than that - an easy pace is always a bit slower than you think. It's also hard to hold that pace - I tend to find myself gradually getting faster if I don't concentrate - so watch that.
Adding easy runs into your week means that, when you do a faster run, your body will use oxygen more efficiently and it will take longer for you to get out of breath.
Not an easy one for most runners but very important.
Don't try to do it all at once i.e. go out and run your target distance at your target pace. You might well be able to do it but you're more likely to pick up an injury and you're at risk of getting demoralised if you don't manage it. It's much better to break it up into smaller chunks - fast one day, long another - so that you get more out of each run, both in terms of enjoyment and improved fitness.
Plan variety into your runs and stick at it. It will take a while for your body to work out that you're no longer just doing the same thing day after day and to adapt. Give it time and you will get great results.