Interval training

Im fairly new to running having only taking it up a year ago. im currertly doing regular half marathons and have worked my time down to 1 hour 24 minutes. I do alot of miles in the week but no interval training and my question is what type of interval training works well?

What do you mean when you say "alot of miles" also do you just do halfs or a variety of distances because speedwork for a 5k is very different to what you would do for half.

I do 3 10 plus mile runs a week along with a 5 mile run to work each day but im staying at similar pace whatever distance i do. If its 3 miles or 15 i always do about 7 minutes 20 miles

So if I have it right your doing about 55 mile per week (mpw). To be honest I think you need to slow down the bulk of your mileage to close to 8min mile pace. You will probably find that you are able to do a couple of your sessions at at faster pace even doing intervals once a week. If your training for a half you'd perhaps do best with mile reps or 800's at a pace slightly faster than your race pace. So for example I've been doing 10x1mile in 6:00 to prepare for the marathon but to be honest most people could do better by just getting more consistent with there training
Doing more miles to increase aerobic conditioning and lastly once you're got as far as you can with consistent miles then add speedwork.
Hope that makes sense

Yeah thank you very much great help, will definatly do that!