根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。
Training for a marathon requires careful preparation and steady, gradual increases in the length of the runs.____, buy the best-fitting, best-built running shoes you can find. No one can say which brand will work best for you or feel best on your feet, so you have to rely on your experience and on the feel of each pair as you shop. When you have found shoes that seem right, walk in them for a few days to double-check the fit.____.As always, you should stretch(仲展) at least ten minutes before each run to prevent injuries.
During the first week, do not think about distance, but run five minutes longer each day.____ it is wise to take a day off to rest. But during the next week, set a goal of at least a mile and a half per run.____.After two weeks, start timing yourself. ____.Depending on the kind of race you plan to enter, you can set up a timetable for the remaining weeks before the race.
A. After six days
B. For a good marathon runner
C. Before you begin your training
D. With each day, increase the distance by a half mile
E. If they still feel good, you can begin running in them
F. Time spent for preparation raises the quality of training
G. Now you are ready to figure out a goal of improving distance and time
(?)