The next thing to do is to figure out what goal pace per mile you need to qualify for the Boston Marathon. Once you figure that out, start adding goal-pace miles to your training. Do this in little segments at first until you get used to it.
Start by doing 3 to 6 miles at goal-marathon pace as your run on any given day. Once you get used to that pace, start adding goal-marathon pace miles at the end of your long runs. For example, if your weekend long run is 16 miles, try doing the final 8 miles at or close to goal-marathon pace.
These miles will be difficult, but getting your body used to running goal pace at the end of long runs will provide the precise physiological adaptations necessary to run at this pace, and will build confidence. For most marathoners, marathon pace coincides with 79 to 88 percent of maximal heart rate or 73 to 84 percent of heart-rate reserve. Start these goal-pace long runs comfortably and gradually pick up the pace during the first half of the run, then run your goal marathon-pace miles at the end.
Many runners make the mistake of doing all of their long runs at one to two minutes slower than marathon pace, then find it difficult to run goal pace come race day. You have to get your body used to running goal-pace miles before race day.