If you are running fairly low mileage currently (and I think this generally applies for most people who are running slower than about 9:30 miles also) you will get faster, and often surprisingly so, just by adding mileage. After a eight weeks of 25-40 mpw, for instance, you will almost certainly be considerably faster than when you started, and I think this comes well before you even start to think about speedwork/intervals/etc. You just want to be careful not to do too much too soon -- be patient and have faith that if you put the miles in, you'll get faster.